THE OFFICIAL Science Questions 2 weeks before the MCAT Thread.

[farley is god]

Thanks in advance for any help.

• how can I determine the crystalline structure of a molecule? i.e. simple cubic, body centered cubic, and face centered cubic?
  
• how is HCF3 able to hydrogen bond? I was under the impression that H had to be bonded to either F,O, or N to be able to form the hydrogen bond.
  
• I f-cking hate Kaplan:
  
  "A physical system is surrounded by a double wall of insulation, and the space between the walls is completely evacuated. By using the vacuum as insulation, which types of heat flow are prevented?"
  
  I. Conduction
  II. Convection
  III. Radiation
  
  A. I only
  B. II only
  C. III only
  D. I, II, and III
  
  shouldn't the answer be I & II ??
  
• more to come, I'm sure. if anyone else has questions I'll do my best to answer 'em.
  

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[fun8stuff]

Originally posted by farleyisgod
Thanks in advance for any help.

• how can I determine the crystalline structure of a molecule? i.e. simple cubic, body centered cubic, and face centered cubic?
  
• how is HCF3 able to hydrogen bond? I was under the impression that H had to be bonded to either F,O, or N to be able to form the hydrogen bond.
  
• I f-cking hate Kaplan:
  
  "A physical system is surrounded by a double wall of insulation, and the space between the walls is completely evacuated. By using the vacuum as insulation, which types of heat flow are prevented?"
  
  I. Conduction
  II. Convection
  III. Radiation
  
  A. I only
  B. II only
  C. III only
  D. I, II, and III
  
  shouldn't the answer be I & II ??
  
• more to come, I'm sure. if anyone else has questions I'll do my best to answer 'em.
  

Is the answer D?

 

[Nuel]

I will say B.

Why? We are under the assumption that the walls aren't insulators. Therefore, via conduction, assuming the walls can conduct heat hence need for insulation, the internal walls' internal energy will increase. By radiation, energy will be transferred to the vacuum; at least not required for the MCAT, bodies above absolute zero emit some form of heat.

The question does not specify the contents of the physical system, and since there are no fluid particles in the insular space (vacuum), we can say convection is the only preventable means of heat transfer. Also, another reason why I eliminated conduction is that it can occur in all three phases: solids, liquids and gases.

 

[Nuel]

how can I determine the crystalline structure of a molecule? i.e. simple cubic, body centered cubic, and face centered cubic?

I don't know, I usually hated these things all the way from high school. It was just another tedium seen again in college. I don't think this is on the AAMC syllabi--I hope so.

off the top of my head: simple cubic--the atoms (or whatever) occupy the corners of the box.

body centered--simple cubic plus a particle at the center of the cube.

face-centered-- simple cubic plus a particle at the center of each of the six faces.

how is HCF3 able to hydrogen bond? I was under the impression that H had to be bonded to either F,O, or N to be able to form the hydrogen bond.

According to Big Fat Kap book, hydrogen bonds form on the F-O-N (Phone)

 

[farley is god]

Thanks for the replies, Nuel.

regarding HCF3,
isn't that a Carbon surrounded by 1 hydrogen and 3 Fluorines?

If so, wouldn't that hydrogen be unable to hydrogen bond?

If not, what's the structure?

Hopefully I'm not missing something obvious, as hydrogen bonding shouldn't be giving me a problem...!

 

[Cerberus]

Christ Farley, thanks for making me realize i dont know crap again

I havent even heard the terms "simple cubic, body centered cubic, and face centered cubic" before

As for the Kap question, my guess was the same as yours

And on the HCF3, I thought C couldnt form hydrogen bonds

 

[Nuel]

My server just shut off . . . dang!

To farley, if I assume right, the C--F bond is pretty polarized and the dipole moment on the compound is not zero due mostly to the electronegativity of F (about 4 by Pauling scale). So we can actually have C--F....H--C hydrogen bond interactions. I would want to think the H-bonds would be weak since the H atom is not directly bonded to some electronegative compound by electronegativity standards, but I can't clearly substantiate that.

Cerb, some of just fake knowledge here but we are haven't scored above a 32 on AAMC .

 

[Nuel]

Farley, have you been able to find the answer to the energy transfer question? I am a little uneasy about my answer.

 

[LUBDUBB]

Basicity of Amines:

Kaplan says that the basicity of amines is increased by electron donating groups. Therefore, tert > sec > primary.


However, don't you have to consider the steric hindrance of tert and sec amines, which results in their reduced solvation. There fore any basicity due to induction is opposed by steric constraints, rendering all amines with generally the same basicity?

Someone call kaplan on their BS.

 

[Cerberus]

Heres one from TPR:

(I think i am just making a simple, yet stupid mistake on this)

The intensity of a sound wave is proportional to the square of its amplitude. If the amplitude of an ultrasonic wave is A at the surface, then at what depth will the amplitude drop by a factor of e.

a) 1/z
b) 2/z
c) sqrt(1/z)
d) sqrt(2/z)

Equation given in passage:
I=I(initial)*e^(-z*d)
where d is depth and z is a positive constant

btw - what kaplan is that "conduction, convection" problem from. I swear i've missed it before!

 

[fun8stuff]

Originally posted by Nuel
I will say B.
Also, another reason why I eliminated conduction is that it can occur in all three phases: solids, liquids and gases.

Yes, and in a vacuum there are no solids, liquids, or gases. This is why I had a hard time eliminating conduction....

 

[farley is god]

Nuel

On reviewing the answer key, the answer to the heat transfer question is unlisted. Here's the answer, verbatim:

"Radiation can travel through a hard vacuum (the Sun?s energy reaches the Earth), so statement
III will be in the correct answer; eliminate choices (A) and (B).
Since there is no matter to either conduct or convect heat, answer choice (C) must be correct."

Unfortunately, Choice (C) would be the opposite of the correct answer, you jackasses.

Cerb.

Don't worry, I have no idea what you know, but I definitely know that I know less than you. And that cubic shape stuff, as Nuel stated, isn't listed on AAMC.

The problem is from Kaplan #8 (which I do not recommend to anyone.) It's not our faults that we missed it.

As for your problem... WTF.

LUBDUBB

I was under the impression that basicity of amines was as follows:

2?>1?>3?

3? being the least basic because of steric constraints. Someone correct me if I'm wrong.

 

[johnnyMD]

Originally posted by farleyisgod


I was under the impression that basicity of amines was as follows:

2?>1?>3?

3? being the least basic because of steric constraints. Someone correct me if I'm wrong. [/B]

correct

 

[fun8stuff]

Originally posted by farleyisgod
Nuel

On reviewing the answer key, the answer to the heat transfer question is unlisted. Here's the answer, verbatim:

"Radiation can travel through a hard vacuum (the Sun?s energy reaches the Earth), so statement
III will be in the correct answer; eliminate choices (A) and (B).
Since there is no matter to either conduct or convect heat, answer choice (C) must be correct."

Unfortunately, Choice (C) would be the opposite of the correct answer, you jackasses.

yeah, answer should be I and II

 

[Drakensoul]

Originally posted by Cerberus
Heres one from TPR:

(I think i am just making a simple, yet stupid mistake on this)

The intensity of a sound wave is proportional to the square of its amplitude. If the amplitude of an ultrasonic wave is A at the surface, then at what depth will the amplitude drop by a factor of e.

a) 1/z
b) 2/z
c) sqrt(1/z)
d) sqrt(2/z)

Equation given in passage:
I=I(initial)*e^(-z*d)
where d is depth and z is a positive constant

btw - what kaplan is that "conduction, convection" problem from. I swear i've missed it before!

I think the hard part about this is interpreting what is meant by "drop by a factor of e." I take it to mean the new value is [(A^2)/e].

I don't know whether or not this is right, though.. comments/corrections appreciated:

I=Io*[e^(-zd)]
[(A^2)/e]=(A^2)e^(-zd)
(1/e)=e^(-zd)
ln1-lne=(-zd)
0-1=-zd
-1=-zd
(1/z)=d

So I'd go with A..

Just for kicks, I tried others..

Such as using [(A^2)-[(A^2)/e]] or [(A^2)-(eA^2)], but the first one comes out to -(1/z)=d, and the second won't give a real answer..

 

[Kazema]

Originally posted by Cerberus
Heres one from TPR:

(I think i am just making a simple, yet stupid mistake on this)

The intensity of a sound wave is proportional to the square of its amplitude. If the amplitude of an ultrasonic wave is A at the surface, then at what depth will the amplitude drop by a factor of e.

a) 1/z
b) 2/z
c) sqrt(1/z)
d) sqrt(2/z)

Equation given in passage:
I=I(initial)*e^(-z*d)
where d is depth and z is a positive constant

Here's my solution:

I = k*A^2 where k is some constant
sqrt(I/k) = A
sqrt ( (I(initial)/k) * e^-zd ) = A

How do you get A to drop by e? by getting -zd = -2, which happens when d = 2/z.

I realize this is a pretty sloppy solution and not the best logic either, so here's my instructor's solution:

I = kA^2
kA^2 = kA(initial)^2 * e^-zd
(A^2)/(A(initial)^2) = 1/ (e^zd)
(A/A(initial))^2 = 1/(e^zd)

Since we want A to equal A(initial)/e,
( (A(initial)/e) / A(initial) )^2 = 1/(e^zd)
1/e^2 = 1/(e^zd)
e^zd = e^2
d = 2/z

 

[Drakensoul]

Originally posted by Drakensoul
....
I=Io*[e^(-zd)]
[(A^2)/e]=(A^2)e^(-zd)
(1/e)=e^(-zd)
ln1-lne=(-zd)
0-1=-zd
-1=-zd
(1/z)=d.....

I see what I did wrong, now..
Final value isn't (A^2)/e, i's (A/e)^2

Which then:
I=Io*e^(-zd)

(A/e)^2 = (A^2)e^(-zd)
[(A/e)^2]/(A^2) = e^(-zd)
(A/eA)^2 = e^(-zd)
(1/e)^2 = e^(-zd)
2ln(1/e)=-zd
2[ln1-lne]=-zd
2[0-1]=-zd
2[-1]=-zd
-2=-zd
d=2/z

So yeah, B
Looks like we're all capable of making simple, stupid mistakes
Thanks, Kazema

 

[fun8stuff]

I came across a question on aamc 5 that asked what gas would be evolved when HNO3 is added to Cu. It ends up H2 gas is not produced, but NO gas is. I think my answer key says something to the effect of the Cu is oxidized so the only way N can be reduced is if NO gas is made. Is this the right explanation? If so, how was I suppose to know that it was an redox problem? All metals and acids are? The thing is that I distinctly remember making H2 gas from HCl and Zn. Thanks for your help.

 

[blz]

Well if you used the mnemonic in the EK book this could have been done by process of elimination. Well first off, CO2 and O3 are definitely not going to be produced so you can eliminate this. In examkrackers is says that the "money metals" (platinum, gold, silver, copper - *except for nickel*) do not spontaneously oxidize in the presence of H+ because they have negative oxidation potentials. Therefore, H2 (2H+ + 2e- = H2; E= 0) should not be produced because the reaction would be nonspontaneous if did and this leaves NO as the only answer. One trend ive noticed in the AAMC exams that they always 2-3 "Bad Boy" questions as I like to refer to them in class. Basically, they are the questions that separate the men (14-15s) from the little girls (<=13). This is an example of a problem that is one of the bad boys of this test. They are usually easy to spot becuase they usually require MCAT knowledge beyond the scope of the MCAT or require you to remeber some miniscule detail. The only way i've been able to consistently get them right is to practice reasoning through them and finding the answer from POE.

Another suggestion I have is to throw away the solution manuals that come with AAMC 3-7. First, I review the test on my own, figuring out the way the MCAT wants me to think and trying to come up with my own tricks to test. KNOW THIS - AAMC DOES NOT WANT YOU TO ACE THEIR TEST. When they give you an explanation to any science problem, they always draw upon indepth scientific knowledge in order to come to the answer. They are not going to give you the hints to trying to figure out the test. So if you see a question in the solution manual that was answered by a page long mathematical anaylsis, DO NOT ASSUME THIS IS THE ONLY WAY! Always try to figure out the shortcuts to any problem and I guarantee you your score will skyrocket. One thing I noticed in my students is that when they saw the explanations in the solution manual, they were scared because AAMC has mentioned scientific knowledge that basically was beyond the scope of the MCAT. So what they did was sit there and try to learn the new material. What they didn't realize was with a little reasoning and process of elimination the answer could have been answered even if you didn't know the indepth science knowledge that AAMC wants you to think you have to know. Remember, do not get scared when you think you see a problem that wasn't covered in a review book - it most likely can be reasoned through. Be confident in the review books because they contain all the information necessary to score a 15. 50% of the MCAT is knowledge you must know, and 50% of the MCAT is learning the test - DO NOT NEGLECT THE LATTER.

 

[fun8stuff]

Originally posted by blz
Well if you used the mnemonic in the EK book this could have been done by process of elimination. Well first off, CO2 and O3 are definitely not going to be produced so you can eliminate this. In examkrackers is says that the "money metals" (platinum, gold, silver, copper - *except for nickel*) do not spontaneously oxidize in the presence of H+ because they have negative oxidation potentials. Therefore, H2 (2H+ + 2e- = H2; E= 0) should not be produced because the reaction would be nonspontaneous if did and this leaves NO as the only answer. One trend ive noticed in the AAMC exams that they always 2-3 "Bad Boy" questions as I like to refer to them in class. Basically, they are the questions that separate the men (14-15s) from the little girls (<=13). This is an example of a problem that is one of the bad boys of this test. They are usually easy to spot becuase they usually require MCAT knowledge beyond the scope of the MCAT or require you to remeber some miniscule detail. The only way i've been able to consistently get them right is to practice reasoning through them and finding the answer from POE.

Another suggestion I have is to throw away the solution manuals that come with AAMC 3-7. First, I review the test on my own, figuring out the way the MCAT wants me to think and trying to come up with my own tricks to test. KNOW THIS - AAMC DOES NOT WANT YOU TO ACE THEIR TEST. When they give you an explanation to any science problem, they always draw upon indepth scientific knowledge in order to come to the answer. They are not going to give you the hints to trying to figure out the test. So if you see a question in the solution manual that was answered by a page long mathematical anaylsis, DO NOT ASSUME THIS IS THE ONLY WAY! Always try to figure out the shortcuts to any problem and I guarantee you your score will skyrocket. One thing I noticed in my students is that when they saw the explanations in the solution manual, they were scared because AAMC has mentioned scientific knowledge that basically was beyond the scope of the MCAT. So what they did was sit there and try to learn the new material. What they didn't realize was with a little reasoning and process of elimination the answer could have been answered even if you didn't know the indepth science knowledge that AAMC wants you to think you have to know. Remember, do not get scared when you think you see a problem that wasn't covered in a review book - it most likely can be reasoned through. Be confident in the review books because they contain all the information necessary to score a 15. 50% of the MCAT is knowledge you must know, and 50% of the MCAT is learning the test - DO NOT NEGLECT THE LATTER.

Thanks for the explanation. My problem is that I didn't recognize that the question wanted more. I answered it based on in the past I have reacted metals and acids and yielded H2. I forgot all about the money metals!

I totally agree with the answer key thing. I always have a quicker solution than they do.

 

[fun8stuff]

Hey Blz you wouldn't happen to have any good advice for verbal, would you? Advice as far as how to better read the passage or better answer the questions. My biggest problem is that every time I take them I feel like I am guessing on every single question and I freak out because I feel like I am getting them all wrong. It is hard going from PS where I know if I answered the question right to verbal where I feel like I guessed on the whole thing. I am stuck getting 9s and 10s.

 

[blz]

Hey I'm in the same boat you're in. I'm getting 14s in PS and BS but only 10-11s in verbal. If anyone else can help us out, please do.

 

[redgrover]

isn't that a Carbon surrounded by 1 hydrogen and 3 Fluorines? If so, wouldn't that hydrogen be unable to hydrogen bond?

We're not assuming a pure solution of CHF3, are we? If not, then CHF3 should be able to hydrogen bond with, say, CH3CH2OH, right? That is, none of the Hs on CHF3 H-bond, but the Fs are capable of hitching up with another H on an OH-group.

 

[fun8stuff]

Okay, I have another question... I have read and heard 2 different things about the speed of sound as it varies according to temperature.

#1
In high school I was always told that sound traveled faster in colder air because it is more dense. Air molecules are closer together (less random motion) and therefore the sound waves pass much faster through cold air, just as sound travels faster in water or through solids than in air. A friend also reassured me of this a few weeks ago.

#2
A while back I read in examkrackers or kaplan or one of those other books that I am using to review with that sound travel faster in warmer air because warmer air has more kinetic energy and more random motion. This allows the sound (vibrations of air molecules) to move/vibrate much quicker through the air.

So, which one do you all think is correct? Thanks a bunch.

 

[IndyZX]

wave speed is dependent on a combination of the inertia and the elasticity of the medium. therefore i think it becomes difficult to make predictions about this sometimes.

more inertia and less elasticity slows waves down
less inertia and more elasticity speeds waves up

 

[Drakensoul]

Originally posted by fun8stuff
Okay, I have another question... I have read and heard 2 different things about the speed of sound as it varies according to temperature.

#1
In high school I was always told that sound traveled faster in colder air because it is more dense. Air molecules are closer together (less random motion) and therefore the sound waves pass much faster through cold air, just as sound travels faster in water or through solids than in air. A friend also reassured me of this a few weeks ago.

#2
A while back I read in examkrackers or kaplan or one of those other books that I am using to review with that sound travel faster in warmer air because warmer air has more kinetic energy and more random motion. This allows the sound (vibrations of air molecules) to move/vibrate much quicker through the air.

So, which one do you all think is correct? Thanks a bunch.

If you're talking about just the speed of sound in air, it travels faster in warmer air than colder.

Speed increases by something like 0.6m/s per degree celcius.

edit: There it is:
v = (331 + 0.610t) m/s, t in Celcius.

 

[Nuel]

Originally posted by fun8stuff
I came across a question on aamc 5 that asked what gas would be evolved when HNO3 is added to Cu. It ends up H2 gas is not produced, but NO gas is. I think my answer key says something to the effect of the Cu is oxidized so the only way N can be reduced is if NO gas is made. Is this the right explanation? If so, how was I suppose to know that it was an redox problem? All metals and acids are? The thing is that I distinctly remember making H2 gas from HCl and Zn. Thanks for your help.

My guess too was H2, and was suspicious of it because it was "too easy". Then I remembered I got a question like this in high school where NO was produced somewhere, but dull memory . . . Shoulda further investigated my hunches.

 

[Nuel]

Originally posted by farleyisgod
Nuel

On reviewing the answer key, the answer to the heat transfer question is unlisted. Here's the answer, verbatim:

"Radiation can travel through a hard vacuum (the Sun?s energy reaches the Earth), so statement
III will be in the correct answer; eliminate choices (A) and (B).
Since there is no matter to either conduct or convect heat, answer choice (C) must be correct."

Unfortunately, Choice (C) would be the opposite of the correct answer, you jackasses.
[

Answer should be I & II. No bodies in contact, no conduction. Clarified.

 

[Idiopathic]

Originally posted by redgrover
We're not assuming a pure solution of CHF3, are we? If not, then CHF3 should be able to hydrogen bond with, say, CH3CH2OH, right? That is, none of the Hs on CHF3 H-bond, but the Fs are capable of hitching up with another H on an OH-group.

You are exactly correct. The F's on this molecule are H-bonding with H's on other molecules.

 

[farley is god]

Originally posted by redgrover
We're not assuming a pure solution of CHF3, are we? If not, then CHF3 should be able to hydrogen bond with, say, CH3CH2OH, right? That is, none of the Hs on CHF3 H-bond, but the Fs are capable of hitching up with another H on an OH-group.

It was pure in the question, and that's what threw me. It should've said aqueous.

 

[Cerberus]

I got the gas question right on 5R but didnt really have a good reason for choosing it over H2 other than slightly remembering the rxn from gen chem.