AAMC CBT8 and 8R OFFICIAL Q&A

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This is the official Q&A thread for AAMC CBT8 and 8R.

Please post ONLY questions pertaining to AAMC CBT8 and 8R.
Out of respect for people who may not have completed the other exams, do not post questions or material from any other AAMC exam.

Please see this thread for the rules of order before you post.

Good luck on your MCAT!

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# 107. I don't understand where the value of 3/2VR came from. I have looked at both Kaplan explanations as well as the aamc's and I don't get it. Please help:)

To make the math easier, let's replace the variables:

P = Blood Pressure
F = CO = Flow rate of blood from the heart (or amount of blood being pumped by the heart)
V = VR = Vascular resistance

so we have:

P = F x V

Now the question says that the pressure is doubled and resistance is increased by 50%, what happens to F (or to the amount of blood pumped by the heart)?
Let's use i to represent the increase or decrease in F...

2P = iF x 1.5V

(2P because pressure is doubled, and 1.5V because resistance increased by 50%-->1+.5)

So solve for i...

(4/2)P = iF x (3/2)V
Divide both sides by (3/2)...

(4/2)/(3/2)P = iF x V
Dividing by a fraction is the same as multiplying by reciprocal

(4/2) x (2/3) P = iF x V
So basically 2 * (2/3), which is (4/3)

(4/3) P = iF x V
Passage established that P = F x V, so (4/3) = i

(4/3) = i
So if the coefficient of F WAS 1, now is 4/3, then it increased by 1/3 (In other words, [4/3] - 1)
 
Hello everyone,

I have a question on the verbal section for AAMC #8.
Here's the question( Its supposed to go along with the behaviorism passage . . .VI):

The passage suggests that behaviorism and type-type identity theory share which of the following weaknesses?

A) They oversimplify mental states by reducing them to physiological states
B) They provide no reason for the existence of pain or other mental states
C) They ignore the context provided by associated beliefs and feelings
D) They assume that all mental events are determined by external physical events

I picked A and the answer was C.
I was actually stuck between these 2 choices ad I know why C is right. I don't know though, why A could be considered wrong. For me, the type-type and the behaviorism seem to be almost the same thing( mental process==> something physical happens) . . . .but the AAMC explanation says they're not the same thing.

Can anyone please clarify?

The talks about how a belief that it is going to rain has its own dispositions (regarding behaviorism). Raining is an external state... mental states = internal. Answer choice A implies that behaviorism and type-type both are subjected only to internal states, which isn't true based on the examples given for behavioral (which would be internal or external).
 
Q 115: The likely genetic basis of increased levels of uric acid is a mutation:

The answer is: affecting an allosteric site of PRPP synthetase.

Can anyone help clarify this? I didn't really understand the AAMC answer

So let's talk about that patient:

w) blood levels exhibited markedly increased levels of 5-PRPP
x) patient has normal levels of PRPP synthetase
y) enzyme activity was three times normal levels in cultured cells
z) pH optimum and the enzyme activity of purified enzyme are normal

Answer choices:
A) affecting an allosteric site of PRPP synthetase
B) affecting the active site of PRPP synthetase
C) in a promoter gene regulating the transcription rate of PRPP
D) a gene coding for a transcription factor for the PRPP synthetase gene

Now, answer choices (C) and (D) would imply that the patient would NOT have normal levels of PRPP synthetase... however since according to information (x), the patient does, those answer choices are eliminated.

So we're choosing between a mutation at the active site or the allosteric site. Mutations typically result in a loss of function, so either we lost function at the active site (which would mean that no product would be formed), or we lost function at the allosteric site (which would mean the enzyme is not being regulated properly).

Loss of function at the active site would typically result in no 5-PRPP being formed, however according to (w) and :thumbup:, that's not the case. Plus (z) tells us that the activity of the enzyme is normal; that is, it isn't more active than it should be in terms of being able to convert substrate into product.

If we lost function at the allosteric site... maybe it acts as an allosteric feedback inhibitor, then that inhibitor or PRPP synthetase activity is not functioning properly. That would result in increased levels of PRPP in the patients as well as in cultured cells.

Answer choice A makes much more sense then the rest based on the information provided.
 
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I don't understand the reasoning for #99 all too well. It says the kinetically controlled product will form faster than the thermodynamically controlled product because the former has a lower activation energy in the reaction profile. But I thought that the reaction profile showed thermodynamic quantities which are independent of reaction kinetics.
 
Bump. Seriously, can I get some help on this?

i believe this is strictly based on memorization, not based on passage information. From what i remember, kinetic products always occurs under low temperature, and it has a lower activation energy and a slightly less stable product; while thermodynamic product occurs under high temperature, which is able to overcome the higher activation energy, and forms a more stable product.
 
Hi, would someone please be able to explain PS #27?

Which of the following items of information would NOT help in predicting the results shown in Figure 1?
A) The number of air molecules inside the balloon
B) The thermal conductivity of the rubber
C) The variation with depth in the speed of the balloon
D) The total mass of the water in the tank

I was able to eliminate A and B right away. From here, I am unsure why C is not the correct answer. I was thinking that the total mass of the water in the tank could help you if you tried a q=MC^T approach to calculate the heat transfer? But apparently that's not the right way to go about it. Would anyone be able to explain please?
 
For PS:

#10

Can someone explain how nucleons(which I take to be protons and neutrons) have magnetic spin? From everything I've read I thought only electrons have magnetics spins?

#25

I understand the concept is Total Internal Reflection, but isn't the fact that this happens due to the glass' REFRACTIVE index? That's the only reason I put refraction instead of reflection..

#27

What does the variation with depth in the speed of the balloon have to do with temperature/depth?
 
I don't understand the reasoning for #99 all too well. It says the kinetically controlled product will form faster than the thermodynamically controlled product because the former has a lower activation energy in the reaction profile. But I thought that the reaction profile showed thermodynamic quantities which are independent of reaction kinetics.

I'm not quite sure what you're confused about, but I'll take a stab at the question...

Regardless of whether the product of the reaction is the kinetic one or the thermodynamic one, it'll have a reaction profile. Think back to collision theory... in reactions, molecules collide and they'll react. However, it was shown that for most reactions, the frequency of collisions was much higher than the number of collisions that resulted in products. It's more than just collisions, you need both molecules to have an adequate amount of energy (activation energy) and they need to be appropriately oriented (spatially). Think Arrhenius equation. The activation energy is independent of the temperature, however, different reaction pathways require different activation energies. Remember, systems that are in equilibrium does NOT mean that reactants have stopped converting to products. It means that the rate of reactants turning to products is EQUAL to the rate of products turning into reactants. This is why thermodynamic products will form (but generally take longer). Because the activation energy is higher for thermo products, the rate at which the products turn back into reactants is considerably smaller.

A.) Irrelevant.
B.) The rate of formation is always dependent on the path.
C.) This is the right answer; the kinetic product typically has the lower energy of activation, which is why it's faster.
D.) Higher energy of activation would mean that it would take longer for the reactant to turn into the product. Think of climbing a 100 foot hill vs. 200 foot hill... it would be quicker to climb the 100 foot hill.
 
Hi, would someone please be able to explain PS #27?

Which of the following items of information would NOT help in predicting the results shown in Figure 1?
A) The number of air molecules inside the balloon
B) The thermal conductivity of the rubber
C) The variation with depth in the speed of the balloon
D) The total mass of the water in the tank

I was able to eliminate A and B right away. From here, I am unsure why C is not the correct answer. I was thinking that the total mass of the water in the tank could help you if you tried a q=MC^T approach to calculate the heat transfer? But apparently that's not the right way to go about it. Would anyone be able to explain please?


The results in figure 1 shows that as depth increases, there is a rise in temperature. The rise in temperature is not from the water itself, because temperature is being held constant. Work is being done on the gas(es) inside the balloon. The depth is representing the amount of pressure experienced by the balloon.

A) The number of air molecules can actually help show how the temperature is related to depth (PV = nRT). This would help us predict the results shown in figure 1.
B) The thermal conductivity of the balloon means how well the balloon would transfer heat. This would be useful information because the rubber is what will allow the conduction of heat from the water molecules to the air molecules.
C) Speed of the ballooon is related to energy by KE = (1/2)mv^2, so here we can relate how the speed of balloon indicates the amount of energy the balloon has (indirectly the temperature). Variation in speed affects KE which affects temperature. Therefore we can relate the temperature AND depth.
D) The total mass of the water would tell you what. You'd know how much mass, you could look up C, but the change in temperature of the water wouldn't be given to you. Even then, the temperature of the water is being held constant by the thermostats and electrical heaters. You certainly wouldn't be able to predict how the depth would affect the temperature. You might make a guess that the air in the balloon would increase, but how would depth play a role? The change in temperature of the balloon is affected by the depth (or pressure). The total mass wouldn't help.

Not sure if this helps.
 
For PS:

#10

Can someone explain how nucleons(which I take to be protons and neutrons) have magnetic spin? From everything I've read I thought only electrons have magnetics spins?

How do you think MRI machine's work? The nucleus of a hydrogen is the same thing as a proton. All subatomic have intrinsic angular momentum (spin). We talk much more about electron spin because of Hund's rule, Pauli exclusion principle, etc, but it's there for all particles. I think the passage implies that protons have spin.
Passage said:
Human tissue contains H atoms; each H atom has a nonzero nuclear magnetic dipole moment, u. (Note: Atomic nuclei with a net spin of zero have a u=0).
I guess you should be asking yourself two things: what are we referring to when we say spin? AND why do electrons have spin? (to be more specific, why do they have intrinsic angular momementum?)

Do a quick google search, the information is there (even though we may not have covered in gen chem).

#25

I understand the concept is Total Internal Reflection, but isn't the fact that this happens due to the glass' REFRACTIVE index? That's the only reason I put refraction instead of reflection..

Every material has a refractive index. For each material's refractive index, there is specific angle called the critical angle... this is the angle at which light is refracted at the a 90 degree angle. Any angle above the critical angle would result in total internal reflection. Refraction is what results in the loss of amplitude. The total internal reflection is what allows the light to bounce off the wall back into the fiber (conserving amplitude). The glass's refractive index would affect the ANGLE. The fact that the glass can have total internal reflection is dependent on it's ability to reflect light.

#27

What does the variation with depth in the speed of the balloon have to do with temperature/depth?

Knowing the speed of the balloon can help you know the kinetic energy of the balloon. Knowing the KE of balloon can help you figure out the temperature (quick google search will give you equation, can't remember right now). Knowing the speed at various depths, will help you plot the temperatures at various depths. See the post above for more in depth commentary on this question.
 
How do you think MRI machine's work? The nucleus of a hydrogen is the same thing as a proton. All subatomic have intrinsic angular momentum (spin). We talk much more about electron spin because of Hund's rule, Pauli exclusion principle, etc, but it's there for all particles. I think the passage implies that protons have spin.

I guess you should be asking yourself two things: what are we referring to when we say spin? AND why do electrons have spin? (to be more specific, why do they have intrinsic angular momementum?)

Do a quick google search, the information is there (even though we may not have covered in gen chem).



Every material has a refractive index. For each material's refractive index, there is specific angle called the critical angle... this is the angle at which light is refracted at the a 90 degree angle. Any angle above the critical angle would result in total internal reflection. Refraction is what results in the loss of amplitude. The total internal reflection is what allows the light to bounce off the wall back into the fiber (conserving amplitude). The glass's refractive index would affect the ANGLE. The fact that the glass can have total internal reflection is dependent on it's ability to reflect light.



Knowing the speed of the balloon can help you know the kinetic energy of the balloon. Knowing the KE of balloon can help you figure out the temperature (quick google search will give you equation, can't remember right now). Knowing the speed at various depths, will help you plot the temperatures at various depths. See the post above for more in depth commentary on this question.


Hey thanks a lot for the in depth responses! One more thing, for the one about magnetic spin:
I know to figure out the net spin of an atom is to see whether it's orbitals are unpaired or not, but that's in terms of electron's quantum #, so in relation to the nucleons, is it simply whether there is an odd number of them?
 
Hey thanks a lot for the in depth responses! One more thing, for the one about magnetic spin:
I know to figure out the net spin of an atom is to see whether it's orbitals are unpaired or not, but that's in terms of electron's quantum #, so in relation to the nucleons, is it simply whether there is an odd number of them?

If a nucleus has an even number of protons AND an even number of neutrons, then it is likely to have a net zero spin. So, yes, if A is odd, then there is a net non-zero spin. If A is even, there could be a net non-zero spin (assuming you had two odd numbers of protons and neutrons).

Check out the isotopes in these tables: http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nspin.html
 
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For BS:

#109
I put ER but for what reasons could one eliminate golgi apparatus since we know it too is involved in packaging secretory proteins?

#110
Are we to always assume that extracting means trying to get the product in the organic layer? I was confused since they didn't say "in order to remove TEA, we should WASH the solution with aqueous.."

#123
Can bacterial conjugation count as sexual reproduction? I got the mitochondria part but in what ways can non-eukaryotes sexually reproduce?
 
Gah, scored two points lower than normal because of verbal.

Anyone else get absolutely murdered by verbal?

EDIT: just figured out why... 4 questions wrong on that damn literature studies passage
 
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Gah, scored two points lower than normal because of verbal.

Anyone else get absolutely murdered by verbal?

EDIT: just figured out why... 4 questions wrong on that damn literature studies passage

yup, same passage, PLUS the one about behaviorism/functionalism. Such vague answers I swear..
 
For BS:

#109
I put ER but for what reasons could one eliminate golgi apparatus since we know it too is involved in packaging secretory proteins?

#110
Are we to always assume that extracting means trying to get the product in the organic layer? I was confused since they didn't say "in order to remove TEA, we should WASH the solution with aqueous.."

#123
Can bacterial conjugation count as sexual reproduction? I got the mitochondria part but in what ways can non-eukaryotes sexually reproduce?

For 109, since the question asks specifically for synthesis, you would have to say ER. For the purposes of the MCAT, the golgi doesn't really synthesize proteins so much as add carbohydrate tags.

There's a couple of ways to look at 110. I think a good way is to see that if you add a strong acid, you will protonate your TEA, driving it into the aqueous layer (because it's charged). This layer can then just be drawn off. If you add a weak/acid base in the other cases, you get some protonated and some deprotonated TEA, which gives some of your TEA in the aqueous layer and some in the organic layer. This isn't an effective way to maximize yield.

For 123, conjugation doesn't count as sexual reproduction because there's no reproduction happening. Conjugation is just a recombination of genes between already existing bacterium - no new units are made. As for the second part, I'd like a good example too of how prokaryotes reproduce sexually. I was under the impression that essentially all reproduced by binary fission.

EDIT: Also, will someone explain #12 in PS? AAMC's explanation does nothing for me.
 
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For 123, conjugation doesn't count as sexual reproduction because there's no reproduction happening. Conjugation is just a recombination of genes between already existing bacterium - no new units are made. As for the second part, I'd like a good example too of how prokaryotes reproduce sexually. I was under the impression that essentially all reproduced by binary fission.

You're right sexual reproduction require meiosis, so bacteria can't sexually reproduce in that sense. However, I think for all MCAT-purposes, we assume that bacterial conjugation (and maybe transformation) is considered as a form of sexual reproduction because they result in genetic recombination.
 
For 109, since the question asks specifically for synthesis, you would have to say ER. For the purposes of the MCAT, the golgi doesn't really synthesize proteins so much as add carbohydrate tags.

There's a couple of ways to look at 110. I think a good way is to see that if you add a strong acid, you will protonate your TEA, driving it into the aqueous layer (because it's charged). This layer can then just be drawn off. If you add a weak/acid base in the other cases, you get some protonated and some deprotonated TEA, which gives some of your TEA in the aqueous layer and some in the organic layer. This isn't an effective way to maximize yield.

For 123, conjugation doesn't count as sexual reproduction because there's no reproduction happening. Conjugation is just a recombination of genes between already existing bacterium - no new units are made. As for the second part, I'd like a good example too of how prokaryotes reproduce sexually. I was under the impression that essentially all reproduced by binary fission.

EDIT: Also, will someone explain #12 in PS? AAMC's explanation does nothing for me.


#12
Passage says at resonance, rotational frequency of B2 equals Wd, then the nucleus can become antiparallel(180 degrees) to B1. You assume it is already parallel to B1 since it precesses at Wd when this condition applies.
 
Will someone also explain #27 in PS? This annoyed me. Wouldn't the mass of the water help you determine the temperature of the balloon? By q = m*c*dT, I would imagine the mass of the water will help us delineate the amount of energy transfer to the balloon. I got a 14 in PS on this today, but questions like this bother me because I was sure it couldn't have been D.

You're right sexual reproduction require meiosis, so bacteria can't sexually reproduce in that sense. However, I think for all MCAT-purposes, we assume that bacterial conjugation (and maybe transformation) is considered as a form of sexual reproduction because they result in genetic recombination.

I still don't see how that constitutes reproduction. Cells recombine their DNA all the time without reproducing. Transposons, which excise themselves out of DNA and insert elsewhere show a good example. And even transduction and transformation show genetic recombination. But in all these cases, the number of cells in equals the number of cells out. There's no reproduction.

EDIT: Thanks Vitamin. I don't see how the hell I missed that.
 
#12
Passage says at resonance, rotational frequency of B2 equals Wd, then the nucleus can become antiparallel(180 degrees) to B1. You assume it is already parallel to B1 since it precesses at Wd when this condition applies.

passage said:
If u is paralell to B1 when B2 begins rotating, the H nucleus will precess at an angular frequency of wd around the direction of B1. IF the rotational frequency of B2 equals wd, the nucleus can become antiparallel to B1.

So basically, if the H nucleus is pointing in the same direction as magnetic field B1, then essentially B2 will flip H so it's pointing in the opposite direction of magnetic field B1. This would imply the nucleus is rotated through an angle of 180 degrees.
 
Will someone also explain #27 in PS? This annoyed me. Wouldn't the mass of the water help you determine the temperature of the balloon? By q = m*c*dT, I would imagine the mass of the water will help us delineate the amount of energy transfer to the balloon. I got a 14 in PS on this today, but questions like this bother me because I was sure it couldn't have been D.

I attempted to give an explanation(s) a few posts above on this.


I still don't see how that constitutes reproduction. Cells recombine their DNA all the time without reproducing. Transposons, which excise themselves out of DNA and insert elsewhere show a good example. And even transduction and transformation show genetic recombination. But in all these cases, the number of cells in equals the number of cells out. There's no reproduction.

Cells recombine their DNA, but do they recombine their DNA with DNA from other cells? I get what you're saying, but think of it like this, bacteria are single-celled, so their genetic recombination has to follow this path.

t0 = bacteria born
t1 = bacteria engages in conjugation
t2 = bacteria goes through binary fission
t3 = bacteria born

They recombine their genetic material at t1. After going through binary fission, you have two new daughter cells (t3). Both of these daughter cells have different DNA than the bacteria at t0.
 
Will someone also explain #27 in PS? This annoyed me. Wouldn't the mass of the water help you determine the temperature of the balloon? By q = m*c*dT, I would imagine the mass of the water will help us delineate the amount of energy transfer to the balloon. I got a 14 in PS on this today, but questions like this bother me because I was sure it couldn't have been D.



I still don't see how that constitutes reproduction. Cells recombine their DNA all the time without reproducing. Transposons, which excise themselves out of DNA and insert elsewhere show a good example. And even transduction and transformation show genetic recombination. But in all these cases, the number of cells in equals the number of cells out. There's no reproduction.

EDIT: Thanks Vitamin. I don't see how the hell I missed that.

#27
Right!? Of all the equations and variables they could give us saying which has nothing to do with change in temp/depth and they say mass instead of the speed of the balloon as it rises?
I can see how we can use the q=mct equation to find the heat exchanged( and only if we knew the heat capacity of the air in the balloon) but it seems even more unlikely to use the speed against the variation of depth, UNLESS we use the speed to see how it accelerates and the increasing acceleration as a correlation of increasing buoyancy force which we can correlate to increased volume and then increased temperature, but that's godddamn crazy and way more variables involved..
 
I attempted to give an explanation(s) a few posts above on this.

I can see how you can equate 1/2mv^2 with perhaps 3/2RT to get temp from speed, but that would only really apply to an ideal and don't you think it would be rather inaccurate since it's a collection of gas particles within a balloon with its own heat transfer properties?
 
I attempted to give an explanation(s) a few posts above on this.




Cells recombine their DNA, but do they recombine their DNA with DNA from other cells? I get what you're saying, but think of it like this, bacteria are single-celled, so their genetic recombination has to follow this path.

Yep. Essentially by definition, transduction involves exchange of fragments of the genome of one cell to that of another via a viral vector. There's no reproduction there.

t0 = bacteria born
t1 = bacteria engages in conjugation
t2 = bacteria goes through binary fission
t3 = bacteria born

They recombine their genetic material at t1. After going through binary fission, you have two new daughter cells (t3). Both of these daughter cells have different DNA than the bacteria at t0.

This is a good example, but ultimately arbitrary. First, fission and recombination are distinct processes. As far as I can tell from reading both TPR and EK books, they aren't dependent on each other. So this example assumes that a bacterium will receive a plasmid through conjugation and consequentially divide. What if it divides and then undergoes conjugation? What if my culture of bacteria is in a stationary phase due to lack of nutrients (and consequentially not dividing), but still undergoing conjugation?

#27
Right!? Of all the equations and variables they could give us saying which has nothing to do with change in temp/depth and they say mass instead of the speed of the balloon as it rises?
I can see how we can use the q=mct equation to find the heat exchanged( and only if we knew the heat capacity of the air in the balloon) but it seems even more unlikely to use the speed against the variation of depth, UNLESS we use the speed to see how it accelerates and the increasing acceleration as a correlation of increasing buoyancy force which we can correlate to increased volume and then increased temperature, but that's godddamn crazy and way more variables involved..

I picked that same choice by POE, because I figured all of the other choices would give us something about the temperature in the balloon. Arg.
 
I can see how you can equate 1/2mv^2 with perhaps 3/2RT to get temp from speed, but that would only really apply to an ideal and don't you think it would be rather inaccurate since it's a collection of gas particles within a balloon with its own heat transfer properties?

Yeah, I wouldn't want to use this to get a definitive number, but I feel like knowing the various speeds at different depths could help you relate depth to temperature. At the very least, you may be able to give a general prediction to the shape of the curve.

To be honest, I didn't like this question.
 
This is a good example, but ultimately arbitrary. First, fission and recombination are distinct processes. As far as I can tell from reading both TPR and EK books, they aren't dependent on each other. So this example assumes that a bacterium will receive a plasmid through conjugation and consequentially divide. What if it divides and then undergoes conjugation? What if my culture of bacteria is in a stationary phase due to lack of nutrients (and consequentially not dividing), but still undergoing conjugation?

Well, does the order really matter? If the bacteria divides and then undergoes conjugation, I would imagine the bacteria would eventually divide again. These hypotheticals may illustrate your point (which I agree with), but it won't change the manner in which the MCAT will grade your answers. Maybe we can agree that the term sexual reproduction has different meanings depending on whether you're referring to eukaryotes or prokaryotes.
 
Well, does the order really matter? If the bacteria divides and then undergoes conjugation, I would imagine the bacteria would eventually divide again. These hypotheticals may illustrate your point (which I agree with), but it won't change the manner in which the MCAT will grade your answers. Maybe we can agree that the term sexual reproduction has different meanings depending on whether you're referring to eukaryotes or prokaryotes.

Lol right my goal isn't to argue which is which, I just wanted to get something solid down because I was under the impression that all bacteria reproduced asexually, by binary fission, and that conjugation was genetic recombination that just happened (so a bacterium could undergo 6 rounds of binary fission before undergoing conjugation once). It'd just be nice to have a solid basis for this so that we can do it the way the AAMC wants lol.
 
I have such a fiery passion against verbal reasoning. This is bull. Question 68, I picked C and here's why:

So C says the relative number of harmless algae will decrease. The relative number. The passage clearly states that if trends continue, all algal species will proliferate, but especially the toxic ones. Therefore, we would expect the relative number of toxic species to increase, and the relative number of nontoxic species to decrease. And yes, I know B is also valid - I was trying to pick between them but felt that I had stronger evidence for C. Fact remains that they are both right.

Now remind me again what this is supposed to be testing? Luck? Because I tend to fail in that subject area.
 
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Some of the explanations to these questions are so half-assed...

For the answer to the gas evolution of carbon dioxide from carbonate and HCl, the explanation is

"HCl reacts with CO3 2- to form CO2. Thus, C is the best answer"

Anyone have an explanation of why HCl reacts with CO3 to form CO2? I figured HCl would just donate a proton to CO2 to form bicarbonate.

I have such a fiery passion against verbal reasoning. This is bull. Question 68, I picked C and here's why:

So C says the relative number of harmless algae will decrease. The relative number. The passage clearly states that if trends continue, all algal species will proliferate, but especially the toxic ones. Therefore, we would expect the relative number of toxic species to increase, and the relative number of nontoxic species to decrease. And yes, I know B is also valid - I was trying to pick between them but felt that I had stronger evidence for C. Fact remains that they are both right.

Now remind me again what this is supposed to be testing? Luck? Because I tend to fail in that subject area.

You and me both. The MCAT is turning me into such an angry person :(
 
Some of the explanations to these questions are so half-assed...

For the answer to the gas evolution of carbon dioxide from carbonate and HCl, the explanation is

"HCl reacts with CO3 2- to form CO2. Thus, C is the best answer"

Anyone have an explanation of why HCl reacts with CO3 to form CO2? I figured HCl would just donate a proton to CO2 to form bicarbonate.



You and me both. The MCAT is turning me into such an angry person :(

H2O + CO2 ---> H2CO3
Reacting CO3 2- with HCl will protonated it to make enough H2CO3 to drive the above equilibrium left. Same reason why high levels of CO2 in blood lead to low pH
 
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H2O + CO2 ---> H2CO3
Reacting CO3 2- with HCl will protonated it to make enough H2CO3 to drive the above equilibrium left. Same reason why high levels of CO2 in blood lead to low pH

That makes perfect sense, thanks!
 
I have such a fiery passion against verbal reasoning. This is bull. Question 68, I picked C and here's why:

So C says the relative number of harmless algae will decrease. The relative number. The passage clearly states that if trends continue, all algal species will proliferate, but especially the toxic ones. Therefore, we would expect the relative number of toxic species to increase, and the relative number of nontoxic species to decrease. And yes, I know B is also valid - I was trying to pick between them but felt that I had stronger evidence for C. Fact remains that they are both right.

Now remind me again what this is supposed to be testing? Luck? Because I tend to fail in that subject area.

Looking back at that question, I didn't even realize that before and that makes a good deal of sense. However, they might have been using relatively in terms of the current population, rather than the population of toxic algae. Still though, I think it is terribly worded because using relative in that context makes it sound much more like you are comparing their numbers proportional to the toxic algae.
 
I have such a fiery passion against verbal reasoning. This is bull. Question 68, I picked C and here's why:

So C says the relative number of harmless algae will decrease. The relative number. The passage clearly states that if trends continue, all algal species will proliferate, but especially the toxic ones. Therefore, we would expect the relative number of toxic species to increase, and the relative number of nontoxic species to decrease. And yes, I know B is also valid - I was trying to pick between them but felt that I had stronger evidence for C. Fact remains that they are both right.

Now remind me again what this is supposed to be testing? Luck? Because I tend to fail in that subject area.

Synapsis, you gotta remember that for VR you gotta think of everything from the author's point of view.
The author in this case is an idiot.
SO, even though you explicitly said algal blooms help all species BUT more so toxic ones, you can't choose the answer which basically repeats this fact! That would require thinking! No no, it must always be the simple answer since this was written by a rather simple man/woman.
 
lol #44 in PS is so wrong. The reactions are literally this:

A-(aq) + B+(aq) = AB(s)

There are no "previously paired ions" being exchanged; the solutions being mixed are just water and a single ion species, and if they weren't the experiment wouldn't make any sense.

AAMC needs to retrain their test writers.

Edit: This was the only question I got wrong in BS / PS. :p
 
Synapsis, you gotta remember that for VR you gotta think of everything from the author's point of view.
The author in this case is an idiot.
SO, even though you explicitly said algal blooms help all species BUT more so toxic ones, you can't choose the answer which basically repeats this fact! That would require thinking! No no, it must always be the simple answer since this was written by a rather simple man/woman.

I'd love to take that advice but it's so conditional. Sometimes it seems like you have to think, and sometimes not, almost regardless of who's writing. Sorry, but I fail to see how that's also not a valid choice because it's a very reasonable and easily-drawn conclusion basic solely on the text. :s
 
lol #44 in PS is so wrong. The reactions are literally this:

A-(aq) + B+(aq) = AB(s)

There are no "previously paired ions" being exchanged; the solutions being mixed are just water and a single ion species, and if they weren't the experiment wouldn't make any sense.

AAMC needs to retrain their test writers.

Edit: This was the only question I got wrong in BS / PS. :p

Agreed. Plus, was I the only one who hadn't heard of the word metathesis?!
 
I have such a fiery passion against verbal reasoning. This is bull. Question 68, I picked C and here's why:

So C says the relative number of harmless algae will decrease. The relative number. The passage clearly states that if trends continue, all algal species will proliferate, but especially the toxic ones. Therefore, we would expect the relative number of toxic species to increase, and the relative number of nontoxic species to decrease. And yes, I know B is also valid - I was trying to pick between them but felt that I had stronger evidence for C. Fact remains that they are both right.

Now remind me again what this is supposed to be testing? Luck? Because I tend to fail in that subject area.

Maybe I got lucky, but I read C and interpreted as actual species dying off rather than the relative populations of toxic and nontoxic species.
 
Agreed. Plus, was I the only one who hadn't heard of the word metathesis?!

Nope, I had never heard of it either.

Speaking of weird questions, the Watson and Crick question at the end of BS felt a little out of place... I wouldn't expect that type of question on a supposed "critical thinking and understanding" exam, since its just regurgitating a historical fact.
 
lol #44 in PS is so wrong. The reactions are literally this:

A-(aq) + B+(aq) = AB(s)

There are no "previously paired ions" being exchanged; the solutions being mixed are just water and a single ion species, and if they weren't the experiment wouldn't make any sense.

AAMC needs to retrain their test writers.

Edit: This was the only question I got wrong in BS / PS. :p

That's the gist of the net ionic equation, but the actual empirical chemical equation is written with two ionic compounds undergoing a double displacement reaction, which is synonymous with metathesis.

EDIT: Plus the other choices make absolutely no sense

I knew the term from middle school chemistry, believe it or not... I think its just an older version.
 
That's the gist of the net ionic equation, but the actual empirical chemical equation is written with two ionic compounds undergoing a double displacement reaction, which is synonymous with metathesis.

EDIT: Plus the other choices make absolutely no sense

I knew the term from middle school chemistry, believe it or not... I think its just an older version.

That simply can't be right. For one, the passage specifies "solutions of aqueous cations" and "solutions of aqueous anions," implying that it's literally beakers of e.g. F- in water and Ca2+ in water being mixed together. Second, if we in fact had soluble ionic compounds dissolved in these starting solutions, each containing a counterion anion for the starting solutions of cations and vice versa, we would not be able to know which metathesis product is the one that precipitated out, (unless we already knew their solubilities, which makes one wonder why this experiment is being done in the first place) and the whole point of the data analysis is that we can definitely say that e.g. CaF2 is insoluble because when we mixed Ca2+ and F- we got a precipitate; not, "when we mixed Ca2+ and F- we got a precipitate so either CaF2 is insoluble or the other metathesis product made of two unknown ions is, oh well lets make conclusions about CaF2!"

It's just ridiculous.

Edit: and yes the other choices make no sense but I chose decomposition because the reaction is at least reverse decomposition whereas there is no metathesis occurring period because there are no partners to be swapped. The question has no correct (or even best) answer as written.
 
I knew it. I believe I encountered it in a TBR passage.

I didn't. I was so proud of getting that question by some hardcore POE. And actually, even if you didn't know what that was, you could rather easily POE it (given that we should for sure know the other three). There really wasn't decomposition occurring, which usually involves more products than reactions. Nothing was being oxidized, and of course hydrolysis reactions involve use of water to to break up reactants. I understand there are more fine ways to look at the options, but at their basic level, I found that B-D couldn't possibly have been correct.
 
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I didn't. I was so proud of getting that question by some hardcore POE. And actually, even if you didn't know what that was, you could rather easily POE it (given that we should for sure know the other three). There really wasn't decomposition occurring, which usually involves more products than reactions. Nothing was being oxidized, and of course hydrolysis reactions involve use of water to to break up reactants. I understand there are more fine ways to look at the options, but at their basic level, I found that B-D couldn't possibly have been correct.

But, as the passage is written, it's not metathesis, so you can POE the question to be anything, such as I did assuming they wanted it construed as "reverse decomposition."
 
That simply can't be right. For one, the passage specifies "solutions of aqueous cations" and "solutions of aqueous anions," implying that it's literally beakers of e.g. F- in water and Ca2+ in water being mixed together. Second, if we in fact had soluble ionic compounds dissolved in these starting solutions, each containing a counterion anion for the starting solutions of cations and vice versa, we would not be able to know which metathesis product is the one that precipitated out, (unless we already knew their solubilities, which makes one wonder why this experiment is being done in the first place) and the whole point of the data analysis is that we can definitely say that e.g. CaF2 is insoluble because when we mixed Ca2+ and F- we got a precipitate; not, "when we mixed Ca2+ and F- we got a precipitate so either CaF2 is insoluble or the other metathesis product made of two unknown ions is, oh well lets make conclusions about CaF2!"

It's just ridiculous.

Edit: and yes the other choices make no sense but I chose decomposition because the reaction is at least reverse decomposition whereas there is no metathesis occurring period because there are no partners to be swapped. The question has no correct (or even best) answer as written.


But how are they to add just bare F- or bare Ca2+? They've got to have some counterion even if it's H+ or OH-, and in the overall reaction it'll look like ion displacement even though since one of the species remains aqueous it can be left out to just have Ca2+ +F- --> CaF2(s)

And even if they had counterions other than H+ or OH- one could understand which precipitate is forming from the other precipitates using the same ions but leaving a common one out etc..
 
But how are they to add just bare F- or bare Ca2+? They've got to have some counterion even if it's H+ or OH-, and in the overall reaction it'll look like ion displacement even though since one of the species remains aqueous it can be left out to just have Ca2+ +F- --> CaF2(s)

And even if they had counterions other than H+ or OH- one could understand which precipitate is forming from the other precipitates using the same ions but leaving a common one out etc..

I don't understand your second paragraph but I can think of several ways to make a net-charged solution of a single ion species solvated in water. Run an electrolytic cell with isolated half-cells at progressively increasing source emf, bubble ionized single-species gas through a salt solution to selectively precipitate one ion species and leave the other, etc. There's no reason a solution has to be charge-neutral, they just usually are. I would have expected solutions of salts to be mixed for such an experiment, but when the passage goes out of its way diction-wise...

The passage very specifically states that these are "aqueous solutions of cations and aqueous solutions of anions." Honestly I see making any further assumption about solution contents to be speculation, and not appropriate given the specificity of the passage information.
 
You're over thinking it wayyyy too much. POE should make it obviously metathesis. There is no way that the passage could indicate any of the other answers.
 
I don't understand your second paragraph but I can think of several ways to make a net-charged solution of a single ion species solvated in water. Run an electrolytic cell with isolated half-cells at progressively increasing source emf, bubble ionized single-species gas through a salt solution to selectively precipitate one ion species and leave the other, etc. There's no reason a solution has to be charge-neutral, they just usually are. I would have expected solutions of salts to be mixed for such an experiment, but when the passage goes out of its way diction-wise...

The passage very specifically states that these are "aqueous solutions of cations and aqueous solutions of anions." Honestly I see making any further assumption about solution contents to be speculation, and not appropriate given the specificity of the passage information.

By the second paragraph I meant after getting a precipitate using 4 different ions, making another solution using 2 of the common ions with 2 other known non-reactive ions and seeing whether it was those two that made the same precipitate.

And yea I get what you mean with making a net charge solution, but in practice you're not gonna have more than a small excess of cation or anion in the solution, not enough to make a solution completely composed of a single, charged ion species.

I understand where the frustration comes from some of these questions(especially in PS). I'm taking pchem atm and some of the explanations they have for entropy or other thermodynamic concepts have the worst use of jargon to justify their answers. That's why everytime I take this I try to rely on just whatever the passage may tell me( and the rather simple interpretation the writer is trying to convey) and what I would know from a basic chemistry/physics course(even though I've gone much more in depth).
 
You're over thinking it wayyyy too much. POE should make it obviously metathesis. There is no way that the passage could indicate any of the other answers.
Haha I'm sorry if 'm being difficult, I promise I'm not trying to overthink this and I really was stumped by the wording when I came to this question during the test. I really just wouldn't ever think metathesis based on what the passage explicitly said. Thus, I wouldn't be able to POE the choices down to metathesis.
By the second paragraph I meant after getting a precipitate using 4 different ions, making another solution using 2 of the common ions with 2 other known non-reactive ions and seeing whether it was those two that made the same precipitate.

And yea I get what you mean with making a net charge solution, but in practice you're not gonna have more than a small excess of cation or anion in the solution, not enough to make a solution completely composed of a single, charged ion species.

I understand where the frustration comes from some of these questions(especially in PS). I'm taking pchem atm and some of the explanations they have for entropy or other thermodynamic concepts have the worst use of jargon to justify their answers. That's why everytime I take this I try to rely on just whatever the passage may tell me( and the rather simple interpretation the writer is trying to convey) and what I would know from a basic chemistry/physics course(even though I've gone much more in depth).

Ah okay, I understand what you mean about solving the precipitation issue, but while that's plausible it also means we would have to make even more assumptions about what the experiment entailed. I just think it's unrealistic to feel justified in assuming so much, especially given the specificity of the wording of the passage. And I absolutely feel you on the entropy deal...
 
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