Vihsadas

No summer
Moderator Emeritus
Lifetime Donor
10+ Year Member
5+ Year Member
Oct 17, 2007
5,469
52
An Igloo
Status
Medical Student
This is the official Q&A thread for AAMC CBT9 and 9R.

Please post ONLY questions pertaining to AAMC CBT9 and 9R.
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!
 

TypeSH07

Junior Member
7+ Year Member
15+ Year Member
Mar 20, 2004
76
0
Visit site
Status
Hi,

Can anybody explain #32 on the PS. Question is "Which of the following statements could explain the frequently bluish color of the EQLs?"

I thought UV radiation absorption would cause if to fluoresce at shorter wavelengths(higher freq) since blue light has high frequency. But the answer is the opposite: flourescing at longer wavelengths.
 

BloodySurgeon

Moderator Emeritus
Lifetime Donor
10+ Year Member
Nov 19, 2006
3,234
13
Status
Medical Student
Hi,

Can anybody explain #32 on the PS. Question is "Which of the following statements could explain the frequently bluish color of the EQLs?"

I thought UV radiation absorption would cause if to fluoresce at shorter wavelengths(higher freq) since blue light has high frequency. But the answer is the opposite: flourescing at longer wavelengths.
Energy absorb = Energy emitted
If Ultraviolet is absorbed it must emit either equal or lower energy light.

IR ROYGBIV UV
from left to right the frequency and energy increases but the wavelength decreases. If UV emitted a shorter wavelength then that would be higher energy. E = hc/wavelength. which is not possible because the energy is not conserved.

However if UV is absorbed it can emit a lower energy radiation like visible light or Blue light. This is the only possible answer because sodium ions makes yellow light and shorter wavelength are better dispersed n=c/(frequency)(wavelength).
 

TypeSH07

Junior Member
7+ Year Member
15+ Year Member
Mar 20, 2004
76
0
Visit site
Status
Energy absorb = Energy emitted
If Ultraviolet is absorbed it must emit either equal or lower energy light.

IR ROYGBIV UV
from left to right the frequency and energy increases but the wavelength decreases. If UV emitted a shorter wavelength then that would be higher energy. E = hc/wavelength. which is not possible because the energy is not conserved.

However if UV is absorbed it can emit a lower energy radiation like visible light or Blue light. This is the only possible answer because sodium ions makes yellow light and shorter wavelength are better dispersed n=c/(frequency)(wavelength).
Oh so its a conservation of energy issue? I automatically thought high frequency b/c of blue light which has almost the highest freq.
 

BloodySurgeon

Moderator Emeritus
Lifetime Donor
10+ Year Member
Nov 19, 2006
3,234
13
Status
Medical Student
the passage states: "SL occurs when bubbles form in the liquid during the rarefaction phase of a sound wave and are then rapidly compressed during the compressional phase of the wave."

It says the the sound wave compressed during the compressional phase. So does that mean that there is another phase? I thought longitudinal wave are just compressed waves.

Also in question 33 it says: Rapid processes are adiabatic, leaving little time for heat to be lost. B and C do not make physical sense. D may be true but is not relevant to the question. Thus, A is the best answer.

But in question 34 it says: Indeed, the compression of the bubble means that work is being done on the bubbles, resulting in the vapor bubbles absorbing heat as their volume goes down. Thus, C is the best answer.

If heat is absorb doesn't that mean the system is not adiabatic? Could someone also please explain these two answers?
 

sicboy188

10+ Year Member
Mar 9, 2008
131
1
Status
Medical Student
the passage states: "SL occurs when bubbles form in the liquid during the rarefaction phase of a sound wave and are then rapidly compressed during the compressional phase of the wave."

It says the the sound wave compressed during the compressional phase. So does that mean that there is another phase? I thought longitudinal wave are just compressed waves.

Also in question 33 it says: Rapid processes are adiabatic, leaving little time for heat to be lost. B and C do not make physical sense. D may be true but is not relevant to the question. Thus, A is the best answer.

But in question 34 it says: Indeed, the compression of the bubble means that work is being done on the bubbles, resulting in the vapor bubbles absorbing heat as their volume goes down. Thus, C is the best answer.

If heat is absorb doesn't that mean the system is not adiabatic? Could someone also please explain these two answers?
34 concerns the amount of heat gained by the compression and 33 concerns the rate of losing that heat to near-by air bubbles. because the compression happens so fast, there is no loss in heat.

note sure if this reconciles the issue, but this is how i understood the question(s).
 

Chemist0157

10+ Year Member
Aug 1, 2007
7,280
293
Status
Attending Physician
Heat doesn't move at the speed of light. Conductions takes some time. I can put my hand on a hot pan for a small fraction of a second without getting burned (cuz I'm quick like that :)).
 

sicboy188

10+ Year Member
Mar 9, 2008
131
1
Status
Medical Student
so i have a question on #117 and its kinda stupid.

Do we know that Peak 1 refers to Compound II and Peak 2 refers to Compound III by comparing the retention times between table 1 and table 2?

while we're on the topic: what exactly is retention time?
 

Chemist0157

10+ Year Member
Aug 1, 2007
7,280
293
Status
Attending Physician
Yep.

Retention time is the time that the compund spends on the column, interacting with the stationary phase. Cmpd III interacts more with the stationary phase and consequently has a longer retention time.
 

sicboy188

10+ Year Member
Mar 9, 2008
131
1
Status
Medical Student
Yep.

Retention time is the time that the compund spends on the column, interacting with the stationary phase. Cmpd III interacts more with the stationary phase and consequently has a longer retention time.
sweet, thanks.
 

Premed Worrier

10+ Year Member
5+ Year Member
Apr 12, 2008
540
0
North Carolina
Status
Medical Student
so i have only taken this one along with 3 and 4.
i was under the impression somehow that number 9 was a relatively difficult one...did anyone else think this was true when they took it?
 

WinterLights

Guest
10+ Year Member
Sep 15, 2007
1,123
2
Status
Pre-Medical
so i have only taken this one along with 3 and 4.
i was under the impression somehow that number 9 was a relatively difficult one...did anyone else think this was true when they took it?
I'd say its on par with six and seven.
 

TawMus

10+ Year Member
5+ Year Member
May 18, 2008
125
0
Status
Resident [Any Field]
I'm not getting the explanation at all. Why is xyulose an isomer of those but not the others..?
 

BloodySurgeon

Moderator Emeritus
Lifetime Donor
10+ Year Member
Nov 19, 2006
3,234
13
Status
Medical Student
I'm not getting the explanation at all. Why is xyulose an isomer of those but not the others..?
isomers are compounds with the same molecular formula but different structural formulae. If you count the carbons hydrogens and oxygens, you will see which are isomers and which are not. i.e. 1-choropentane and 3-chloropentane are isomers with the same molecular formula but they are structurally different because you have to break a bond to reform the other structure.
 

RySerr21

i aint kinda hot Im sauna
10+ Year Member
Dec 22, 2007
5,931
26
Status
Fellow [Any Field]
edit: i still dont get their explanation but i understand the concept so thats all that matters...... nevermind!
 

FourMoreYears

10+ Year Member
May 15, 2008
66
0
Fl
Status
Medical Student
Question #2 PS
If the speed of the charged particle described in the passage increased by a factor of 2, the electrical force on the particle would:

The answer says it will remain the same.

I thought the formula was F=qVBsin0
force=charge (velocity aka speed) (magnetic field)

I thought that since the speed increased by 2 the force would then increase by 2.

Where did I have a brain fart?
 

BTC

10+ Year Member
7+ Year Member
Mar 7, 2008
244
2
Status
Medical Student
There is no magnetic field so no force is generated on the particle because of its velocity.

Question on #29 with the pulley. Why is their equal amounts of work when lifting the object and when lowering the object? When lifting you are going against gravity and in lowering you are working with gravity.

Solution

Mark Which action involves more work: lifting a weight from A to B or lowering the weight from B to A?

A
) Lifting from A to B B
) Lowering from B to A C
) Equal work in both actions
The work done is mgΔh, and because none of these values changes in magnitude when the mass goes up or down, these actions involve the same amount of work. Thus, C is the best answer.


D
) No work is required using a pulley.
 

Premed Worrier

10+ Year Member
5+ Year Member
Apr 12, 2008
540
0
North Carolina
Status
Medical Student
Question #2 PS
If the speed of the charged particle described in the passage increased by a factor of 2, the electrical force on the particle would:

The answer says it will remain the same.

I thought the formula was F=qVBsin0
force=charge (velocity aka speed) (magnetic field)

I thought that since the speed increased by 2 the force would then increase by 2.

Where did I have a brain fart?
remember that the force of an electrical field does not depend on the speed of the particle...the formula you have is for magnetic field...this was my reasoning anyway
 

maluskeeter

10+ Year Member
7+ Year Member
Dec 28, 2007
22
0
Status
#50: Which of the following equations correctly represents the dissolution of NH4NO3(s) in water?

A:
D
) NH4NO3(s)
NH4+(aq) + NO3–(aq)


I don't see how Im supposed to know that they just dissosciate under water conditions. I feel like Im missing a key concept here?
 

Kaustikos

Archerize It
10+ Year Member
Jan 18, 2008
12,205
4,163
Always Bespin
#50: Which of the following equations correctly represents the dissolution of NH4NO3(s) in water?

A:
D
) NH4NO3(s)
NH4+(aq) + NO3–(aq)


I don't see how Im supposed to know that they just dissosciate under water conditions. I feel like Im missing a key concept here?
I think this was testing your knowledge on solubility and how certain compounds dissociate in water. By using process of elimination, however, one can narrow down the choices to C and D. A is wrong because none of them are ions. B is wrong because of the charge configuration is wrong for how the two compounds bonded. C is incorrect because that isn't how a compound would dissociate in H2O.
 

koopa_troopa

Junior Member
10+ Year Member
7+ Year Member
Mar 28, 2005
288
0
Status
Medical Student
#50: Which of the following equations correctly represents the dissolution of NH4NO3(s) in water?

A:
D
) NH4NO3(s)
NH4+(aq) + NO3–(aq)


I don't see how Im supposed to know that they just dissosciate under water conditions. I feel like Im missing a key concept here?
From what I remember, anything with a NO3 will dissolve following the formula:

ANO3 --> A+(aq) + NO3-(aq)
 

maluskeeter

10+ Year Member
7+ Year Member
Dec 28, 2007
22
0
Status
145. Why are high concentrations of sodium included in the dialysate (Table 1)?

i understand the explanation BUT are we expected to know that the plasma contains 28–140 mEq/L of sodium??

Thats the only way you can answer this question

thanks
 

SamuraiPanda

10+ Year Member
5+ Year Member
May 28, 2008
23
1
Status
For #105, how the heck are we supposed to understand what they mean by "Semilogarithmic"? I have no idea why that answer is correct, nor why how they could possibly expect us to know a concept like that.
 

creekfreek24

10+ Year Member
Nov 7, 2007
106
0
Status
Medical Student
For #105, how the heck are we supposed to understand what they mean by "Semilogarithmic"? I have no idea why that answer is correct, nor why how they could possibly expect us to know a concept like that.
i think you can simplify this one panda...answer choice D is the only graph that shows the Y-axis in a logarithmic scale...ie, it's increasing by a factor of 10 everytime. i got that wrong and then was kicking myself afterwords. thats all i got...if anyone wants to give a better explanation please go ahead.
 

mytoechondriac

10+ Year Member
Jun 14, 2008
66
0
Status
PS #2

remember that the force of an electrical field does not depend on the speed of the particle...the formula you have is for magnetic field...this was my reasoning anyway

Why can't we set the kinetic energy= qXEXd? I thought the force would increase by a factor of 4 when the v is doubled.
 

sportstownusa

10+ Year Member
7+ Year Member
Jul 18, 2007
148
0
Status
Medical Student
i have the CBT 9 exam, but dont have the solns. I would really appreciate it if someone who has the solns (for either the CBT or the paper version) would be able to pm it to me.
 

BerkReviewTeach

Company Rep & Bad Singer
Vendor
10+ Year Member
May 25, 2007
3,902
684
i have the CBT 9 exam, but dont have the solns. I would really appreciate it if someone who has the solns (for either the CBT or the paper version) would be able to pm it to me.
You are asking someone to violate their user agreement with AAMC and potentially jeopardize their admissions to medical school. Not to sound too much a pissed-off old terd, but you should consider paying AAMC for access to their exams and solutions.
 
  • Like
Reactions: DrDreams

Deepa100

Junior Member
10+ Year Member
Aug 24, 2006
1,027
1
Status
i think you can simplify this one panda...answer choice D is the only graph that shows the Y-axis in a logarithmic scale...ie, it's increasing by a factor of 10 everytime. i got that wrong and then was kicking myself afterwords. thats all i got...if anyone wants to give a better explanation please go ahead.
I agree, process of elimination is the only way you could solve this one. But D looked soo incorrect!
 

Deepa100

Junior Member
10+ Year Member
Aug 24, 2006
1,027
1
Status
Does any one have a better explanation for this one than the AAMC? In a pendulum, KE is highest at the middle and PE is maximum at the two ends where amplitude is maximum. So, for the wave in Q, if the amplitude is high at either end, shouldn't the velocity be zero?
 

Deepa100

Junior Member
10+ Year Member
Aug 24, 2006
1,027
1
Status
My TPR book says speed of sound decreases with density and increases with resistance to compression.

So, since iron is denser than water, shouldn't the speed of sound be lower?
 

Deepa100

Junior Member
10+ Year Member
Aug 24, 2006
1,027
1
Status
145. Why are high concentrations of sodium included in the dialysate (Table 1)?

i understand the explanation BUT are we expected to know that the plasma contains 28–140 mEq/L of sodium??

Thats the only way you can answer this question

thanks
My reasoning was that you don't want water to be lost from the blood. Hence the dialysate must be isotonic w.r.t the blood.
 

Chemist0157

10+ Year Member
Aug 1, 2007
7,280
293
Status
Attending Physician
My TPR book says speed of sound decreases with density and increases with resistance to compression.

So, since iron is denser than water, shouldn't the speed of sound be lower?
That doesn't sound right. The closer particles are to one another, the faster sound waves can move throughout them...I thought it was the opposite.
 

HawkeyePostOp

the fort at sidewalk
10+ Year Member
5+ Year Member
Jun 4, 2008
729
0
34
a long and winding road
Status
My TPR book says speed of sound decreases with density and increases with resistance to compression.

So, since iron is denser than water, shouldn't the speed of sound be lower?
Nope. TPR is right but you don't really need to know this. Also, TPR's CBT book doesn't explain it.

The equation for speed of sound is c = (coeff of resistance to compression / density)^(1/2)

Iron is about 7 times more dense than water, which on its own, would slow sound.

BUT, Iron also has a bulk modulus (resistance to compression) of about 1.6 E11 N/m^2. Water is about 2.2 E9 N/m^2, so Iron's is about 70 times more resistant to compression.

iron is faster

try playing with the numbers
 
Last edited:

Deepa100

Junior Member
10+ Year Member
Aug 24, 2006
1,027
1
Status
Nope. TPR is right but you don't really need to know this. Also, TPR's CBT book doesn't explain it.

The equation for speed of sound is c = (coeff of resistance to compression / density)^(1/2)

Iron is about 7 times more dense than water, which on its own, would slow sound.

BUT, Iron also has a bulk modulus (resistance to compression) of about 1.6 E11 N/m^2. Water is about 2.2 E9 N/m^2, so Iron's is about 70 times more resistant to compression.

iron is faster

try playing with the numbers
Thanks for the clarification, but...
None of these values are provided in the question statement. So, for questions like this, is it safe to just go with speed of sound is higher in solids than in liquids or gases?
 

HawkeyePostOp

the fort at sidewalk
10+ Year Member
5+ Year Member
Jun 4, 2008
729
0
34
a long and winding road
Status
So, for questions like this, is it safe to just go with speed of sound is higher in solids than in liquids or gases?
In just about all cases, yes. But since AAMC is finding weirder things to trip people on, who knows. I'm not totally sure about wood and composite materials. If the passage has gone out of its way to ask you a question like this, they might hide the numbers implicitly in a paragraph.
 

CommSNTS

10+ Year Member
Jan 6, 2008
37
0
Status
Pre-Medical
Question on PS 49:

A 7-N force and an 11-N force act on an object at the same time. Which of the following CANNOT be the magnitude of the sum of these forces?

A) 2N
B) 8N
C) 12N
D) 18N

Says the answer is A, but if both of them act at a 60 degree angle, 5.5 N (from the 11N force) and 3.3 N (from the 7 N force) in opposite directions equals 2. Are they not taking angles into account?
 
Last edited:

Chemist0157

10+ Year Member
Aug 1, 2007
7,280
293
Status
Attending Physician
Look at the two extremes. If they are acting in the same direction at 0 degrees, the result is 18 N (maximum). If they are acting in opposite directions at 0 degrees, the result is 4 N (minimum). 2 N is the only answer that does not fall within these bounds.

I actually got this question wrong because I had no idea we could think in those terms.
 

dazed1980

10+ Year Member
Dec 3, 2006
232
0
Status
Non-Student
On AAMC 9 #28, can someone explain why bombarding Be9/4 with Deuterien
H2/1, with emission of a neutron led to B10/5 instead of B11/5?

Sorry, i don't know how to superscript those numbers.
 

dazed1980

10+ Year Member
Dec 3, 2006
232
0
Status
Non-Student
Also, AAMC 9 #3. why would changing a resistor to a lower resistance not increase the capacitance. Since C=Q/V and V=IR i though decreasing resistance, would decrease volatge, thereby increasing capacitance.
 

dwc929

10+ Year Member
Apr 20, 2008
310
0
Status
Pre-Medical
Still confused about 33 even after reading above...can anyone explain?
 

tco

10+ Year Member
Apr 22, 2008
2,115
845
Status
Resident [Any Field]
I'm taking this practice test right now...The VR section is BRUTAL.
 

dwc929

10+ Year Member
Apr 20, 2008
310
0
Status
Pre-Medical
Also, how come in passage 7 of physical...how come the diverging lens in figure 3 has the final rays converging?
 

tco

10+ Year Member
Apr 22, 2008
2,115
845
Status
Resident [Any Field]
It doesn't. It wants to know what the rays do after they hit the lens. I missed that one but it makes perfect sense now. :-\

I'm not too upset with my performance. 13 PS 10 VR 11 BS I did sorta cheat, however...

During the VR, some ***, *** , ******** **** (use your imagination) came into the library and made a lot of noise. I stopped taking the test and left for about 5 minutes to avoid a confrontation...I came back and moved and started over again with the passage I was distracted from.

I also didn't complete the second writing sample...

Add:

Is doubling time for an organism a constant value? I missed number 104 because I didn't think you could extrapolate information from a one hour section of time (between 6 and 7 hours) for the whole graph. It's not a constant slope so I don't see how that works...
 
Last edited:

Adnama

10+ Year Member
Jun 11, 2008
104
2
Status
Pre-Medical
Also, AAMC 9 #3. why would changing a resistor to a lower resistance not increase the capacitance. Since C=Q/V and V=IR i though decreasing resistance, would decrease volatge, thereby increasing capacitance.
I know you posted this in the wrong thread... but I will answer anyways. Hope that is okay.

C = eo A/d Capacitance C only depends on A and d. Although Q=CV this equation only tells us the relationship between Q and V. If you have any capacitor, and don't know Q or V - you could still figure out C.

Think of Capacitance as the measure of the plates 'capacity' to hold charge at a given voltage.
 

Quantum Chem

10+ Year Member
Jun 10, 2008
91
0
College Park, Md
Status
Pre-Medical
On AAMC 9 #28, can someone explain why bombarding Be9/4 with Deuterien
H2/1, with emission of a neutron led to B10/5 instead of B11/5?

Sorry, i don't know how to superscript those numbers.
You're adding 2 (one proton and one neutron) to 9, one proton to the total amount of protons 4. Then you are removing one neutron from the total amount of neutrons and protons to leave B10/5.

Remember that the top number is the total number of protons and neutrons and the bottom number is just the amount of protons.
 
OP
Vihsadas

Vihsadas

No summer
Moderator Emeritus
Lifetime Donor
10+ Year Member
5+ Year Member
Oct 17, 2007
5,469
52
An Igloo
Status
Medical Student
Hi Dazed,
Hope you find the MCAT forums useful.

In the future, please heed matt's suggestion to post your questions in the thread that is appropriate for the particular AAMC test that you have questions about. If you see the stickies at the top of the forum, you'll see the thread:

AAMC Official Practice Exam Q&A Discussion Threads & Rules of Order.

In that thread, you'll find a list of threads to post your questions for each different AAMC exam. The reason we like to have all of the AAMC exam questions sequestered to these threads is so that students who may not have taken one or more of the AAMC exams won't accidentally click on the thread and see the answers to an exam they haven't completed.

Thanks!
I'll go ahead and merge this thread with the official threa for AAMC #9.
 

cornpops21

10+ Year Member
May 25, 2008
266
0
Status
Medical Student
I have a question about #12 on the Physical Sciences.

I understand their explanation that ETOH would evaporate the fastest b/c it has the highest vapor pressure. But what I DON'T understand is how it has the highest vapor pressure??

If you look back at #7, ETOH is the molecule that can undergo hydrogen bonding...which should increase its boiling point....so shouldn't it be reasonable to conclude that ETOH would boil off last??