The Official June 2015 MCAT Thread

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drrao88 said:
If anyone wants to take a stab at another tough one:


In the electrolysis of water shown below, a current of 2 amps is applied to 180 mL of H2O(l) for 6 hours and 42 minutes. How many grams of H2(g) are formed? (Faraday’s constant = 96,500 C/mol)

2 H2O(l) + 2 e– → H2(g) + 2 OH–(aq)

A. 10 g
B. 5.0 g
C. .5 g
D. .25g
Click to expand...
a?
 
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any chance that a should be 1 gram and not 10? I keep redoing it and keep getting 1 gram as my answer
 
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drrao88 said:
If anyone wants to take a stab at another tough one:


In the electrolysis of water shown below, a current of 2 amps is applied to 180 mL of H2O(l) for 6 hours and 42 minutes. How many grams of H2(g) are formed? (Faraday’s constant = 96,500 C/mol)

2 H2O(l) + 2 e– → H2(g) + 2 OH–(aq)

A. 10 g
B. 5.0 g
C. .5 g
D. .25g
Click to expand...

I got C. Found the charge transferred over that time period (2 C/S * (402 minutes *60 seconds/minut)) = ~48000C. Using Faraday's constant = 0.5 moles of e-. You need 2 moles of electrons per mole of H2 gas produced, so 0.25 moles of H2 produced x its molecular weight, so 0.5g. What was the answer??
 
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soccergurl1987 said:
I got C. Found the charge transferred over that time period (2 C/S * (402 minutes *60 seconds/minut)) = ~48000C. Using Faraday's constant = 0.5 moles of e-. You need 2 moles of electrons per mole of H2 gas produced, so 0.25 moles of H2 produced x its molecular weight, so 0.5g. What was the answer??
Click to expand...
Agreed. I think C
 
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Answer is C.

The total moles of electrons is .5, and to find out the number of moles of H2 it will reduce, you have to divide by 2. (2 moles of electrons to go from H+ to H2), so this comes out to .25 moles of H2 reduced, which equals .5g H2
 
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MD.Hopeful said:
Testing on Saturday, got 90% in Bio AAMC question pack, 88% in chemistry, 87% in physics today.

Is that enough to get me 510+ on the actual thing? God I hope so...
Click to expand...
I don't think the question packs mean all that much in terms of score conversions but those scores show you know the material so I think you will be fine
 
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drrao88 said:
Answer is D.

The total moles of electrons is .5, and to find out the number of moles of H2 it will reduce, you have to divide by 2. (2 moles of electrons to go from H+ to H2), so this comes out to .25 moles of H2 reduced, which equals .25g H2
Click to expand...

Isn't 0.25 moles of H2 = 0.5g? (H2 = 1*2???) Am I missing something?
 
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Lol. I mixed up my Q from TPR to what he wrote. It is C .5g as explained by:
"First, convert the time to seconds: (6 hours × 3600 seconds/hour) + (~40 minutes × 60 seconds/minute) to get ~24,000 seconds. Then, multiply by the current of 2 amps to get the total charge applied: ~48,000 C. Using Faraday’s constant, this is equivalent to ~0.5 moles of e–. Since we have 10 moles of H2O, H2O is in excess and e– are the limiting reagent. For every 2 moles of e– used, 1 mole of H2 is formed, which means 0.25 moles of H2 are formed in this problem. Multiply by the molecular weight of H2 (2 g/mol) to get 0.5 g H2. Overall:
9682_Q1142915_1.gif

Note that the question is asking for the number of grams of H2, not the number of moles (choice A is wrong). Also note that if we used H2O for our calculations, we would have arrived at 5 moles of H2 (choice C) and 10 grams of H2 (choice D)."
 
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drrao88 said:
Crap, you are right. Wow, I think I need to stop studying when I cant figure out what 2 x .25 is.
Click to expand...

Lol, I think we're all getting to that point. I keep catching myself making stupid math mistakes on problems, or misreading questions. Really hope I don't do that too much on the real thing.
 
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ONE MORE!

I heard the MCAT isn't going heavy on these kinds of questions anymore, but I have a feeling these concepts will still pop up.:


A ball is being swung in a circular motion with radius 3 m. The tension in the string is found to be equal to 2 times the weight of the ball when the ball is at the top of it's circular orbit. What is the speed of the ball if the string breaks at this point?

A. 3√g
B. √3g
C. 9√g
D. 3g
 
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drrao88 said:
ONE MORE!

I heard the MCAT isn't going heavy on these kinds of questions anymore, but I have a feeling these concepts will still pop up.:


A ball is being swung in a circular motion with radius 3 m. The tension in the string is found to be equal to 2 times the weight of the ball when the ball is at the top of it's circular orbit. What is the speed of the ball if the string breaks at this point?

A. 3√g
B. √3g
C. 9√g
D. 3g
Click to expand...
a
 
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drrao88 said:
ONE MORE!

I heard the MCAT isn't going heavy on these kinds of questions anymore, but I have a feeling these concepts will still pop up.:


A ball is being swung in a circular motion with radius 3 m. The tension in the string is found to be equal to 2 times the weight of the ball when the ball is at the top of it's circular orbit. What is the speed of the ball if the string breaks at this point?

A. 3√g
B. √3g
C. 9√g
D. 3g
Click to expand...

Not too sure about this one. I got the sqrt(3g). Is that what b is? My rationale was that Fnet = T - mg (so mg), and that Fc=mv^2/r, so when you eliminate m from either side of the equation, and isolate v you get sqrt(3g).

I sure hope there's no circular motion stuff on the exam though. I haven't looked at that stuff at all since it wasn't on the outline.
 
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soccergurl1987 said:
If I could remember them, maybe I'd despise them 😛

I kid. I don't mind them too much. I really hate this psych/soci section in general though. So much pure memorization required, rather than understanding.
Click to expand...
Singer schacter, cannon bard, james lang? my god.
 
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drrao88 said:
ONE MORE!

I heard the MCAT isn't going heavy on these kinds of questions anymore, but I have a feeling these concepts will still pop up.:


A ball is being swung in a circular motion with radius 3 m. The tension in the string is found to be equal to 2 times the weight of the ball when the ball is at the top of it's circular orbit. What is the speed of the ball if the string breaks at this point?

A. 3√g
B. √3g
C. 9√g
D. 3g
Click to expand...

A?
 
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soccergurl1987 said:
Not too sure about this one. I got the sqrt(3g). Is that what b is? My rationale was that Fnet = T - mg (so mg), and that Fc=mv^2/r, so when you eliminate m from either side of the equation, and isolate v you get sqrt(3g).

I sure hope there's no circular motion stuff on the exam though. I haven't looked at that stuff at all since it wasn't on the outline.
Click to expand...

Oh, I think you're right! Totally forgot to -mg to get Fnet
 
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How in depth did you guys go with the Biochem regarding Fats?

Did you memorize all the steps of synthesis/oxidation?

And what about all the enzymes of glyco-genesis/genolysis?!

I hate how the AAMC is so vague about this stuff on the content outline. Just tell us wtf we need to know if you really want this new test to emphasize "more on how we can apply what we know"
 
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drrao88 said:
How in depth did you guys go with the Biochem regarding Fats?

Did you memorize all the steps of synthesis/oxidation?

And what about all the enzymes of glyco-genesis/genolysis?!

I hate how the AAMC is so vague about this stuff on the content outline. Just tell us wtf we need to know if you really want this new test to emphasize "more on how we can apply what we know"
Click to expand...

I didn't go too in depth, but I generally know what happens. Like I know for fatty acid synthesis, it goes from acetyl CoA to manoyl (spelling?) CoA then to a fatty acid but that's as far as I know.
 
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stressingout said:
Just out of curiosity, how did you guys feel about the CARS on the official guide vs the CARS on the sample test?
Click to expand...

I thought the cars on the sample test was easier than usual. I did way better than I usually do. It's probably partially luck, but my score was about 10% higher than it usually is on other AAMC material.
 
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soccergurl1987 said:
I thought the cars on the sample test was easier than usual. I did way better than I usually do. It's probably partially luck, but my score was about 10% higher than it usually is on other AAMC material.
Click to expand...

Okay, I was just asking because my CARS score on the guide was way higher than my CARS score on the sample test so I was wondering if it was because the guide was easier or because I improved
 
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drrao88 said:
How in depth did you guys go with the Biochem regarding Fats?

Did you memorize all the steps of synthesis/oxidation?

And what about all the enzymes of glyco-genesis/genolysis?!

I hate how the AAMC is so vague about this stuff on the content outline. Just tell us wtf we need to know if you really want this new test to emphasize "more on how we can apply what we know"
Click to expand...

Doesn't seem very necessary to understand in depth steps for FAS/degradation. If you know an overview, you should be fine. For example, I highly doubt they will be asking something specific about carnitine transport regarding FA's, unless something in the passage highlighted steps or there were cues in there where you could make a link. However, they may want you to know how much more energy a FA has in comparison to say glucose. But even then, I could imagine them showing the steps for degradation and you would only be responsible for knowing how much ATP is produced based on the NADH (~2.5)and FADH2 (~1.5) that is produced during B-oxidation.

Same may be true with glycolysis/neogenesis. However, these steps seem to be higher yield as knowing them is such an integral part of many classes and is key to understanding how cell's meet their energy demands.

You should be more concerned with drawing conclusions based on fundamentals. I.e if Pyr is in Xs, glycolysis and B-Oxi is likely to be down regulated. Lack of glc and ATP is low? Well, it's safe to assume B-Oxi will be up regulated due to X, Y, and Z.
 
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alexisfrozen said:
Doesn't seem very necessary to understand in depth steps for FAS/degradation. If you know an overview, you should be fine. For example, I highly doubt they will be asking something specific about carnitine transport regarding FA's, unless something in the passage highlighted steps or there were cues in there where you could make a link. However, they may want you to know how much more energy a FA has in comparison to say glucose. But even then, I could imagine them showing the steps for degradation and you would only be responsible for knowing how much ATP is produced based on the NADH (~2.5)and FADH2 (~1.5) that is produced during B-oxidation.

Same may be true with glycolysis/neogenesis. However, these steps seem to be higher yield as knowing them is such an integral part of many classes and is key to understanding how cell's meet their energy demands.

You should be more concerned with drawing conclusions based on fundamentals. I.e if Pyr is in Xs, glycolysis and B-Oxi is likely to be down regulated. Lack of glc and ATP is low? Well, it's safe to assume B-Oxi will be up regulated due to X, Y, and Z.
Click to expand...
n3s2u.jpg
 
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haha, i was just looking at bernulli, sound, electromag, biochem, and it terrified me just looking at the equations because i cant remember what half of it means.
Also can anyone explain why Km changes with inhibitors and enzymes.
 
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libertyyne said:
haha, i was just looking at bernulli, sound, electromag, biochem, and it terrified me just looking at the equations because i cant remember what half of it means.
Also can anyone explain why Km changes with inhibitors and enzymes.
Click to expand...

Competitive = inhibitor fights with substrate to bind at enzyme active site. Based on this competition, the "affinity" of the enzyme is lowered because it doesnt always bind the way it wants to (e.g. substrate binds 5 times out of 10 instead of 10 out of 10)

Noncompetitive = Km stays same because the inhibitor binds away from active site, so the affinity of the enzyme is not impacted

Uncompetitive = Inhibitor only binds after the enzyme-substrate complex has been formed, so again Km is not impacted
 
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Okazaki Frag Grenade said:
Competitive = inhibitor fights with substrate to bind at enzyme active site. Based on this competition, the "affinity" of the enzyme is lowered because it doesnt always bind the way it wants to (e.g. substrate binds 5 times out of 10 instead of 10 out of 10)

Noncompetitive = Km stays same because the inhibitor binds away from active site, so the affinity of the enzyme is not impacted

Uncompetitive = Inhibitor only binds after the enzyme-substrate complex has been formed, so again Km is not impacted
Click to expand...
Sweet thanks!
 
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Okazaki Frag Grenade said:
Competitive = inhibitor fights with substrate to bind at enzyme active site. Based on this competition, the "affinity" of the enzyme is lowered because it doesnt always bind the way it wants to (e.g. substrate binds 5 times out of 10 instead of 10 out of 10)

Noncompetitive = Km stays same because the inhibitor binds away from active site, so the affinity of the enzyme is not impacted

Uncompetitive = Inhibitor only binds after the enzyme-substrate complex has been formed, so again Km is not impacted
Click to expand...

I think Km decreases for uncompetitive because affinity increases. Thats how EK explained it at least.
 
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alexisfrozen said:
However, they may want you to know how much more energy a FA has in comparison to say glucose.
Click to expand...

Doesn't this depend on its size i.e. how many Ac-CoA's you can chop out (along with maybe a propionyl CoA)? So without context it's hard to know exactly "how many times more" energy this has got than a glucose molecule. We can definitely say it's more, though!


alexisfrozen said:
You should be more concerned with drawing conclusions based on fundamentals. I.e if Pyr is in Xs, glycolysis and B-Oxi is likely to be down regulated. Lack of glc and ATP is low? Well, it's safe to assume B-Oxi will be up regulated due to X, Y, and Z.
Click to expand...

In total agreement on this. The calculation of ATP based on the NADH and FADH2 standards you used (2.5 and 1.5, respectively) may be a stretch though, considering some use 1 for FADH2 and 2/3 for NADH. I think they'd either need to explicitly define it (where you have to know how many of those molecules are produced to get it) OR they'd have to have an answer that is close the answer you would get using either of the values for those molecules, with the other answers being too far away to matter. Either way I'm using 1.5/2.5. Seems to be the safest bet.
 
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For the official guide questions, I kept checking my answers after every question .. Probably not the best way to study :/
 
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Okazaki Frag Grenade said:
Uncompetitive = Inhibitor only binds after the enzyme-substrate complex has been formed, so again Km is not impacted
Click to expand...

You're right that uncompetitive inhibitors bind to the ES complex, but Km decreases (Kaplan 2015 Biochem, pg. 54, table 2.2) because the uncompetitive inhibitor "locks" the enzyme into the substrate, which effectively means the enzyme's affinity for the substrate has increased.
 
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