MCAT Question of the Day

Correct Answer: C. Charles’s gas law

Charles’s Law states: V T
Dalton’s law of Partial Pressures states: Ptotal = P1 + P2 + P3 + …
Boyle’s gas law states: P 1/V
The Ideal gas law states: PV = nRT

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Correct Answer: C. keystone predator

The otter would be considered a SECONDARY consumer, the urchins are the primary consumers, and a hypothetical predator of the otter would be a tertiary consumer. Since the otter is responsible for preventing overgrazing of the small grasses by predating on the out of control urchin population, it is termed a keystone predator that exerts a “top-down” effect on the ecosystem.

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Correct Answer: A. Electric current will only flow from a high potential to a lower potential.

Electric current moves in the direction positive charges move in. Therefore, the electric flow of current flows from the point of higher potential to lower potential, similar to how water falls from the top of the waterfall to the bottom.

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Correct Answer: A. 1-cyclopropyl-4-isopropyl-1,3-dimethylcyclohexane

1. First, identify the (longest) parent chain or cycle -> six carbon ring = “cyclohex”

2. If several substituents are on the ring, they are listed in alphabetical order, and the first substituent is given #1 priority. The cyclohexane has the following substituents: cyclopropyl (#1), 2 methyl groups, iso-propyl (note that iso is not considered when placing in alpha order)

3. Substituents are numbered such one of them is at carbon #1 and the other locations have the lowest possible numbers, going in either a clockwise or counter-clockwise direction, giving the prefix di-, tri-, tetra-, to groups of the same kind (prefixes are not considered when alphabetizing).

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Correct Answer: B. the equilibrium mixture has more reactants than products

The chemical equilibrium constant is calculated by dividing the concentration of the products by the concentration of the reactants at equilibrium. The magnitude of the chemical equilibrium constant can shed light on the composition of the mixture at equilibrium. Since the equilibrium constant given in the question is small, the equilibrium mixture would contain more reactants than products. If the equilibrium constant were larger than 1, this would indicate that the equilibrium mixture was made up of more products than reactants. Note: the chemical equilibrium constant is not an indicator of the rate of the reaction, only the concentrations of the equilibrium mixture.

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Correct Answer: C. Diastereomers

When drawing out the structures, one can see that the basic skeletal arrangements are identical, i.e. stereoisomers (not constitutional isomers). We can dismiss conformers, since the structures cannot be interconverted exclusively by rotations about formally single bonds. The compounds are diastereomers, a class of stereoisomers that are non-superimposable, nonmirror images of one another. More specifically, the compounds are epimers: in chemistry, epimers are diastereomers that differ in configuration of only one stereogenic center.

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Correct Answer: D. Asymmetric induction

Racemization refers to the conversion of an optically active substance to a racemic form (has equal amounts of left- and right-handed enantiomers of a chiral molecule). Enantioenrichment refers to the act of enriching an already existing mixture of stereoisomers such that it contains more than 50% of one enantiomer. Chiral chromatography involves the separation of stereoisomers using a chiral stationary phase, which does not involve chemical transformations. “Optical reduction” as a technique or phrase does not exist.

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Correct Answer: C. convex.

The meniscus is the curved upper surface of liquid. This is formed because of unequal cohesion and adhesion attractions. For example, in water the adhesive forces are greater than the cohesive forces, therefore the water is able to “walk up the sides” of the surface. In the case of water, a U-shaped or concave meniscus is formed. Mercury, for example has stronger cohesive attractions between the mercury molecules than adhesive attractions between the mercury and the glass surface, therefore producing a convex meniscus, which looks like an inverted U.

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Correct Answer: C. 2H2O2 -> 2H2O + O2

The second law of thermodynamics states that nature tends to favor a state of disorder or entropy. In other words, chemical reactions that are exergonic (release energy) and result in more products than reactants are favored. The only answer option that has more products formed in comparison to the reactants is answer option C.

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Correct Answer: A. CH4, NH3, H2, H2O

In the primitive atmosphere, no O2 was present as most organisms used chemosynthesis to create organic compounds. The Oparin and Haldane hypothesis states that methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O) were the only gases present. It was after the Miller and Urey experiment that it was proven that these gases and lightning could have given rise to simple organic compounds that then later evolved into more complex forms of life.

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