FL 9 - 1, 21, 41

I had a couple questions from the physical sciences section of Full-Length 9. The first one is actually #1. I chose answer c, and I still can't understand why the answer is b. Wouldn't the elastic part be easier to stretch? My next question is number 21, I just can't put together an equation-based relation that includes velocity. Also, could you explain number 41? Thank you!!
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Question 1:

For this question, we need to analyze the graph.  The graph is stretch versus time (NOT force), for a system with a viscous component and an elastic component in parallel.  Well, if we give a certain CONSTANT force to a spring over time, what happens?  It stretches until it reaches a certain deformation from equilibrium, and then just stays there.  Think about hanging a mass from a spring – that’s a constant force on the spring which causes deformation until the mass is at translational equilibrium.

What would happen to a viscous object without an elastic force?  Well, it would stretch and stretch and stretch… and keep stretching until (probably) it eventually broke.  But either way, it doesn’t reach equilibrium because it’s not elastic.  So it just keeps stretching.

Why is it asymptotic, then?  Well the spring will only stretch until it’s in equilibrium with the force.  At that point, it won’t stretch anymore.  Thus, the stretch of the viscous component (which should be infinite) is limited by the stretch of the spring.

Question 21:

We’re supposed to relate the force created by the spring with this scenario of a pilot landing a jet.  Well, anytime you see force, think acceleration!  F_net = ma, but F also represents the force of the spring here, and thus F_spring = kx.  So, we can say F_spring = ma.  We’re given one more key piece of information:  the final velocity is zero.  Acceleration is simply change in velocity per time or Δv/t.  Since v_f = 0, we can say that a = (v_f – v_i)/t = (0 - v_i)/t = -v_i/t.  Plugging into our equation, we get F_spring = m(-v_i/t) = -mv_i/t.  Thus, the force is directly proportional to the starting velocity.

Question 41:

The first thing we see when looking at the answers is the fact that it says that one has higher or lower energy than the other, and differences in principle quantum number (5sto 5d, etc.).  So, let’s start with energy.  E = hf, so the higher the frequency, the higher the energy.  Red light has the lowest frequency while violet has the highest, so that means red light has lower energy than violet.  Here, we’re dealing with red (Sr) and green (Ba).  So that means that strontium has a lower energy jump than barium does.

Well, for principle quantum number, where is strontium?  It’s in the 5s grouping.  Barium, however, is 6s.  Where are these electrons jumping to?  Well, it should jump to the next-highest-energy orbital.  If we start at an s orbital, we’ll just jump to p or d.  So we’ll want to see that electrons jumping from 5s to 5p or d have less energy than those jumping from 6s to 6p or d.  And that matches with answer choice (B).  

AAMC #3, BS 114

I'm having a hard time understanding why developing a leak in the apparatus increases the surface pressure and thus increases the BP of both substances. How do we know the pressure in the appartus is different  from atmospheric pressure to begin with?

When the leak occurs does the vapor pressure of the liquids also decrease because temperature is going to be decreasing? This wouldnt necessarily mean the BP will be different, just that it will take longer to get there right?
Thanks!
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This question reads:  "If a leak develops in the vacuum distillation apparatus, the boiling points of the two components of caraway seed oil will:"

The key, then, is the fact that it says "vacuum distillation."  Remember that vacuum distillation will lower the pressure above the two liquids being distilled, so that it lowers their boiling point.  This is because liquids will boil when their vapor pressure equals the ambient pressure.  By lowering the ambient pressure, liquids will boil at a lower temperature because they don't need as high of a vapor pressure.  Vacuum distillation is used for liquids that have boiling points >125 degrees Celsius.

Thus, if a leak develops, we will expect the pressure inside the apparatus to go up.  It starts lower than atmospheric pressure, because that's the whole goal of vacuum distillation.  When a leak develops, the pressure will go up because air comes in from the surrounding atmosphere.  By increasing the ambient pressure, the boiling point starts going up again.

The vapor pressure of the liquids shouldn't change here just because of the leak.  Indirectly, it will cause them to go up in the end.  Remember that boiling is an isothermal process.  By increasing the boiling point, we increase the vapor pressure needed to cause that boiling.  Thus, assuming that we're supplying heat to the apparatus, the temperature will go up because the boiling point has gone up.  Before, it would hit the boiling point and stop at that temperature since the boiling process is isothermal.  Now, it adds heat until the boiling point -- at a higher temperature and higher vapor pressure -- and then boils at this temp.

Scaled scores

Does scaling the scores according to the percentage correct vary from exam to exam? I've just been noticing differences with the percent I got correct and my scaled score- like on FL 5, my scaled score for physical sciences was 2 points higher than what the percentage I got correct would have said.
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I've said this a few times before -- please be careful with the rubric I gave you in the MSCT III packet.  It gives you an idea of where your score falls, but it is not wholly accurate for the MCAT.  The MCAT varies from test administration to test administration; likewise, our scaling varies from Full-Length to Full-Length.  The scaled scores you are given by the Kaplan tests are quite accurate; it wouldn't be fair for us to either give you a false sense of confidence or to make you feel like you're not doing as well as you actually are.

The scaled score conversion chart should be used from section tests, and remember that it predicts usually +/- 1.  In your case, it was off by 2, but usually it should be accurate -- give or take 1 scaled score point.

Cyclic ring substituents

If you have a cycloalkane or cycloalkene with 2 substituents, so either way you number gives the same low numbers, how do you decide which gets the lowest number in the nomenclature? for example, what if a chlorine and methyl are 1 carbon apart?
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Assuming equal priority for all the substituents, you want the lowest number to be on the substituent that would come first alphabetically.  Thus, a cyclohexane with a chlorine and methyl group one carbon apart would be 1-chloro-2-methylcyclohexane.

However, if there is a group with higher priority (say there was an alcohol on that same cyclohexane), that should ALWAYS be given the lowest possible number.

Bio III Questions

1) How does diffusion (e.g. through a cell membrane) compare to what occurs at the glomerulus (why can glucose easily pass through even though particle is charged)? Can urea passively pass through by diffusion for both a cell membrane and the glomerulus? Is filtration purely based on charge?
Diffusion will only allow the passage of small, nonpolar, uncharged molecules (besides water) through the cell membrane.  Filtration in the kidney, however, allows through those particles PLUS some biomolecules (amino acids, glucose, vitamins being the noteworthy ones), a number of ions (Na+ and Cl- being the main ones), and some other waste products.  This is because the glomerulus has much larger pores, which permit through these larger molecules.  Urea requires a carrier protein, but no energy, to pass through a cell membrane as well as the collecting duct and ascending loop of Henle.  It doesn't really show up significantly in the glomerulus.  And filtration isn't based on charge at all; it's based on size.

2) Hb sat curve shift: why isn’t the Hb saturation curve the same directional shift as for a person on a mountain (mountain dweller or mountain goat) as it is for a fetus (in both cases less O₂ present)? If this is the case, why would a mountain dweller like the case for muscles, why would O₂ need less binding while exercising? Hypothetically, what would happen if no shift, how would muscles be impacted?
Let's think about this numerically.  Let's say you're standing in air with 50 mmHg of oxygen (half the normal amount).  Let's say that there's no shift.  Well, looking at the curve, you'll have about 80% saturation.  Then, in the muscles (around 20 mmHg), you'll have about 20% saturation.  So, in the lungs, the blood is carrying 50 mmHg * 80% = 40 mmHg of oxygen.  Then, in the muscles, the blood is carrying 20 mmHg * 20% = 4 mmHg.  How much went into the muscles?  Well, the difference:  40 mmHg - 4 mmHg = 36 mmHg.

With the shift, let's say we accomplish 100% saturation at the 50 mmHg of oxygen, and have 30% saturation in the muscles.  We then have 50 mmHg * 100% = 50 mmHg in the lungs, and 20 mmHg * 30% = 6 mmHg on the blood in the muscles.  Thus, we've given the difference of 50 mmHg - 6 mmHg = 44 mmHg to the muscles.  This may not seem like a huge difference to the muscles, but it's still an improvement!

3) What does the MCAT use as their definition of sense and anti-sense strands (since the definition found in the foundation review contradicts the one given in the review notes)?
The "sense" strand is the strand of DNA NOT paired with mRNA during transcription (although, because of base-pairing, it will have the identical sequence of nucleotides to the mRNA but with T switched for U).  The "antisense" strand is the one base-pairing with the mRNA.

4) For differentiating amides and steroids, we stated in class the need for a carrier protein to carry steroids through the blood stream given its lipophilic character. When the steroid passes approaches the cell membrane is it released from the carrier, and the after enter the cell does it once again pick up another carrier since the cytosol/cytoplasm is polar environment? How are T3/T4 able to wiggle past through the cell membrane when both are amides and why do they not require a carrier protein if they have the ability to pass through the lipid membrane? Finally, typically, when referring to thyroid hormone, is T4 the prevalent form in the blood stream?

I have to admit that I'm not sure about the carriers inside the cell for steroid hormones; my guess would be that they exist, but I'm not sure.
T3 and T4 are usually grouped as amino-acid derived hormones, which is true; however, since they're derived from tyrosine and each features two benzene rings and are quite small, they can get through the cell membrane.  On the other hand, they do have a polar amide on one end and a number of iodine atoms, which help them dissolve in the aqueous environment of the blood.  The generic term "thyroid hormone" would refer to both T3 and T4 as a group, not just one of them.

PS Test 2 - 32 and 39

I had trouble understanding the explanations for 39 and 32 on the Physical Science Section Test 2, because it seemed like they contradict each other, but I don't know if that's because 32 regards a galvanic cell and 39 is about an electrolytic cell? Anyway, I understood the explanation for 32 to be saying that the EMF would increase since more electrons are flowing and the reaction rate is increasing. But then the explanation for 39 says that doubling the rate at which electrons are produced will not affect the potential difference.

Thanks!
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Ultimately, these two questions boil down to thinking about voltage.  You can think of voltage as how intense the draw is on each electron, sort of like a force.  Voltage itselfisn't a force, but it's an analogy that helps explain these two questions.

In the case of question 32, when we increase the concentrations of our reactants, that means that we'll have our reactions at our cathodes and anodes happening more often, or at a faster rate.  Thus, at any given time, we start building up even more attractive force for the electrons at our cathode, and even more repulsive force from our anode.  That is, our anode gets more negatively-charged and our cathode more positively-charged at any given time, since the reaction is going quicker.  This means that we've increased the "force" or voltage on our electrons.

In 39, however, we haven't actually changed the effect on any given electron.  Yes, we have more flowing, but each one experiences the same pull it did as when we have the original current.  With the same "pull" as before, we have the same voltage.

Density/Specific Gravity Question from AAMC #4

Hi Alex,

 

Sorry to bother you again-but this question from AMCAS MCAT # 4 is driving me insane and I think it's because I'm overthinking it. So, here goes:

 

18) An object with 15 grams mass is immersed in benzene and suffers an apparent loss of mass of 5 grams. What is the approximate specific gravity of the object? (Data: Specific gravity of benzene = 0.7)

 

a) 1.4

b) 1.8

c) 2.1

d) 3.0

 

The answer is (c).
Solution given: The buoyant foce on an object immersed in a fluid is equal to the weight of the fluid displaced by the object (Archimedes' principle). There were 5 g of liquid displaced; thus, the ratio of object mass to fluid mass is 15/5 = 3. The specific gravity of the object (mass per unit volume compared to water) is three times the specific gravity of benzene (3 x 0.7= 2.1) because the volumes of object and displaced liquid are equal.

 

*I don't understand why the ratio of the mass of the object to the mass of the liquid is 3. Where are they getting these numbers? Please explainnn :(

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When we approach this question, the first thing that should hopefully stick out is that we're dealing a bit with buoyancy.  We put the object in a fluid and the weight changes; what is responsible for that change?  The buoyant force, of course.

So let's think about what forces are acting on the object before it's in the fluid.  We have a force of gravity equal to the mass of the object times the acceleration due to gravity.  That's really the only one we're concerned about in that case.

When we put the object into the fluid, the new net force is less.  The "apparent loss of mass" here is due to the buoyant force, and must be equal to five grams times the acceleration due to gravity (why?  Because if the apparent mass is now 10 grams, then the apparent weight would be 10 times the acceleration due to gravity.  We've lost 5 grams times the acceleration due to gravity, which is completely attributable to the buoyant force).

Remember that, by definition, the buoyant force is equal to the weight of the fluid that's been displaced.  It is also equal to the density of the fluid times the acceleration due to gravity times the submerged volume of the fluid (in this question, since they use the word "immersed," we know the full volume has been submerged).  We care about the volume here, because we can use that to calculate the density of the submerged object.  If we know the submerged object's volume (which must be equal to the volume of displaced fluid, logically), and its mass (provided in the question stem), we can figure out its density and thus figure out its specific gravity.

Our calculations would look like this:

(rho of fluid)(g)(V)=(mass of fluid)(g)

Taking out g on both sides, we have:

(rho of fluid)(V)=(mass of fluid).

Note that this is basically just a reiteration of the density equation.

From there, let's plug in:

(0.7)(V)=5 grams

and V thus equals 5/0.7.

Taking that one step further, we can say that if the volume of the cube must also be 5/0.7, and its mass is 15 grams, then its density is 15/(5/0.7) = 3*0.7 = 2.1.  That gets us to answer choice C.