MCAT Question of the Day

Correct Answer: A. A

An SN2 reaction is a concerted process and does not involve intermediates (ruling out profile C and D). Also, reaction transition states are higher energy than the starting material leaving profile A as the only option.

Recall that 2nd order reactions can be written of the type: rate = k[RX][Nucleophile] and so the total of the exponents = 2 (thus 2nd order overall = 1st order with respect to RX and 1st order with respect to the nucleophile).

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Correct Answer: B. B.

The polarization of a bond is determined by the relative electronegativity of each particular atom. As Cl is more electronegative than a hydrogen atom, so the dipole moment points towards the Cl atom along the H-Cl bond. As a good memory guide, from your physics review, you would know that an electric field line always points away from a positive charge.

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Correct Answer: A. CH3CO2H

Recall that pKa = -log Ka so their relative values, because of the negative sign in the equation, are always opposite to each other. Thus in an acid base equilibrium, AH <--> A- + H+, the largest pKa value is associated with the weakest acid. The unhalogenated carboxylic acid A is the weakest acid (acetic acid is the IUPAC name; ethanoic acid is the systematic way to name it). In the other cases, the electron withdrawing halogen substituent stabilize the carboxylate conjugate base (anion) making the parent acid more acidic.

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Correct Answer: D. continue moving with constant velocity.

For something to change direction or accelerate there must be a net force acting on it (Newton’s 2nd Law).

Also, Newton’s 1st Law states that an object in motion will stay in motion at constant velocity (a form of inertia) unless acted upon by a net (an unbalanced) force.

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Correct Answer: D. Skeleton

The mesoderm develops into muscle and connective tissue which includes blood and bone (skeleton!), as well as circulatory, reproductive and excretory organs. The ectoderm produces the epidermis and the nervous system including the brain. The endoderm will become the epithelial lining of the respiratory tract, and the digestive tract including the glands of the accessory organs (i.e. the liver and pancreas). The stage of the embryo where the 3 primary germ layers first develop is called the ‘gastrula’.

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Correct Answer: B. B

While in the area between the plates, the negatively charged electron is attracted to the positive plate and so it would bend towards the positive plate, or “upwards” in the diagram. But after leaving the plates, there is no more net force acting on the electron. Therefore, the electron would continue in motion in a straight line according to Newton’s first law (inertia).

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Correct Answer: A. CH3COO^–

Carboxylic acids are considered to be among the strongest organic acids but in comparison with all types of acids, carboxylic acids are very weak. By far, the protonated form of answer choice A (acetic acid is the IUPAC name; ethanoic acid is the systematic way to name it) would be the weakest of that group (after all, we are comparing it with nitric acid, sulfuric acid and phosphoric acid).

Logically, a strong acid would create a weak conjugate base, otherwise the acid would not give up the proton readily. Thus the reverse is true: a weak acid has a strong conjugate base.

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Correct Answer: C. A, H, Q, B

Genetic linkage relates the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. Genes whose positions (= loci) are nearer to each other are less likely to be separated onto different chromatids during chromosomal crossover, and are thus genetically linked.

If there is very little recombination (see Q and H in the table) then those loci must be very, very close to each other. If the percent recombination is high, it suggests that the loci are further apart and that’s why they assort independently more often. In other words, the percent recombination underlines how far away loci are on the gene map.

A -> 16 -> B
A -> 10 -> Q -> 6 -> B
A -> 5 -> H -> 5 -> Q -> 6 -> B

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Correct Answer: D. 11 tons.

This is the kind of reasoning question that you can expect as a result of an MCAT passage.

The difference in pressure between inside and outside the cabin at 10,000 meters is:
75 – 20 = 55 kPa = 55,000 Pa or N/m² of pressure pushing on the door (keep in mind that the SI unit for pressure is the pascal, Pa).

Pressure is defined as force per unit area: P = F/A
The area of the door is 2 x 1 = 2m²

So F = PA = (55,000)(2) = 110,000 N
Thus weight mg = 110,000, and since g = 10 m/s², then m = 11,000 kg or 11 metric tons.

Solving this problem requires reasoning, dimensional analysis and one very basic equation. List of physics equations to memorize: www.goldstandard-mcat.com/physics-equation-lists/

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Correct Answer: B. B.

The primary amines A and B can donate two hydrogen bonds and accept one (due to the lone pair on N). Generally, for compounds with the same functional group, the higher molecular weight compounds have a higher boiling point due to increased opportunity for intermolecular van der Waals (weak, but better than nothing) interactions. Of course, even though a periodic table is available during the exam with a click of a button, you do not need to calculate the molecular weight since you can quickly determine the number of carbons of the different N-containing structures which easily reveals B as the “heaviest.”

Helpful hint: If all other factors are the same, increased molecular weight means that it is harder to push that molecule into the vapor phase meaning the boiling point would be higher.

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