1,600+ Free MCAT Practice Questions, Organized by Section and Topic

Correct Answer: Choice C

This question asks how one pH treats two proteins that differ in their point of zero net charge. We must use our knowledge of charge and solubility to answer this question.

The isoelectric point is the pH at which a protein carries no net charge, and it is also the pH of its minimum aqueous solubility. Away from that point every molecule in the solution carries the same sign of charge, so neighbours repel one another and stay dispersed.

Treat the two proteins separately. The first has an isoelectric point of 4.6, so in this buffer its net charge is zero, the repulsion between molecules disappears, and it aggregates and falls out of solution. The second has an isoelectric point of 8.2, so a buffer more than three units below that value leaves it strongly protonated and net positive. Those molecules repel one another and remain dissolved. Selective precipitation of this kind is why acidifying milk to about pH 4.6 curdles casein and leaves the whey proteins behind.

(Choice A) The second protein sits far from its isoelectric point, so it keeps the charge that holds it in solution.

(Choice B) The first protein has no net charge here, so nothing prevents its molecules from aggregating.

(Choice D) This reverses the two proteins, since the buffer matches the isoelectric point of the first.

Correct Answer: Choice D

This question asks which hydrostatic quantity depends on the shape of a vessel. We must use our knowledge of pressure and force to answer this question.

Pressure at a depth is the product of fluid density, gravitational field strength, and depth alone. No area and no shape appear in it, so three vessels filled to the same depth with the same fluid have identical base pressures no matter how they flare or taper. A narrow vessel holding a litre and a wide one holding twenty litres press equally hard on every square centimetre of their bases.

The force is a different quantity and follows from the first. Force is pressure multiplied by the area over which it acts, so with the pressure fixed the force scales directly with the base area. The three forces are therefore unequal, and the widest base carries the largest one.

This pairing is the hydrostatic paradox: the force on the base can exceed the weight of the fluid in the vessel when the walls slope outward, because the walls push down on the fluid as well.

(Choice A) Unequal pressures would require the depths or the densities to differ, and the stem fixes both.

(Choice B) This reverses the pair, letting shape affect the pressure and not the force.

(Choice C) The pressures are right, but equal pressures on unequal areas cannot give equal forces.

Correct Answer: Choice B

The stem names the heuristic and asks what follows from it. The task is to work out which correction operates on the input the heuristic actually uses.

The availability heuristic estimates how common something is from how easily instances come to mind, so the currency of the judgement is retrieval rather than evidence. Two diagnoses from last week are highly retrievable, which is why the rate feels high.

A correction can therefore work only by changing what comes to mind. Bringing several patients to mind who had the same presentation and a different diagnosis loads the retrieval set with competing instances, and the felt frequency of the rare disease falls with it. The physician does not have to revise any belief for that to happen, which is what makes it the prediction the heuristic itself generates.

(Choice A) A printed prevalence figure is a statistic, not an instance. It can be read and accepted while the impression of frequency survives, which is why base rates so often fail to correct availability.

(Choice C) Being told the cases were unusual is an argument about them. It leaves them just as easy to recall as they were before.

(Choice D) Rereading the notes rehearses the two cases and makes them more retrievable, so availability predicts the estimate rises rather than falls.

This question wants us to find what the passage as a whole is doing. We can use the main idea of the passage to answer this question. The main idea is that paper claims cannot exceed the surplus a nation can produce. To answer this question, we will eliminate the answer choices that do not align with this idea.

A is incorrect. This answer choice is incorrect because it names the belief the author sets out to overturn. Paragraph 1 says the settlement rests on the assumption that the old order "was a natural condition to which Europe will return of its own accord," and then announces that "the assumption is mistaken." Restating a belief he immediately attacks cannot be his concern. We can eliminate A.

B is incorrect. This answer choice is incorrect because it takes one step for the destination. Paragraph 2 does describe the two habits, one belonging to the labouring population and one to the owning class. That description supports the larger claim about payment rather than standing as the point. This is the tempting choice, because paragraph 2 is the most vivid part of the passage. We can eliminate B.

C is incorrect. This answer choice is incorrect because it is stronger than anything the author says. Paragraph 6 states "I do not dispute the justice, and I have no wish to be understood as disputing it." He objects to a demand that cannot be met, not to reparations as such. We can eliminate C.

D is correct. This answer choice is correct because it is most in line with the author's main idea. Paragraph 1 says that when the assumption falls "the arithmetic falls along with it," and paragraph 4 explains why: a claim is definite on paper, but "The surplus must first exist." The sums were written for an economy that no longer stands.