MCAT Question of the Day

Correct Answer: A. 1, 4

Rotation of plane polarized light is indicative of a chiral molecule. If not, the compounds must be meso or achiral. Compounds 1 and 4 are meso compounds, which, despite having two or more stereocenters, are not chiral (superposable on its mirror image). Compounds 2 and 3 are chiral molecules.

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Correct Answer: B. tert-butyl carbocation

The most stable fragment likely formed from 2,2-dimethylpentane fragmentation is t-butyl species.

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Correct Answer: D. Benzoic acid, an organic compound, is insoluble in water. Extraction into water can be achieved with aqueous sodium hydroxide (deprotonation -> polar sodium benzoate, now soluble in water). Transfer of the benzoate back into organic solution is accomplished by treatment with hydrochloric acid (protonation -> less polar benzoic acid).

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Correct Answer: B. 1/16

F1 generation is the heterozygous offspring of the homozygous parents. F2 generation is the offspring of the heterozygous offspring generation F1. The ratio is 9:3, 3:1 in this generation, meaning that only one of the offspring will have black fur and a long tail.

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Correct Answer: B. Atomic radius increases

As you move from left to right across a Period, metallic properties decrease (metals > metalloids > non-metals), atomic radius decreases (the atoms get smaller), electronegativity (the ability to attract electrons in a compound) increases, first ionization energy (the energy required to remove the outermost electron form a neutral atom) increases and electron affinity (the energy released when an electron is bonded to an atom) increases.

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Correct Answer: B. Equilibrium favors the products.

In an acid-base equilibrium such as this, the relative strength of an acid can be determined using the magnitude of Ka (or pKa): as Ka increases, the strength of the acid increases; as pKa (inverse log of Ka) decreases, the strength of the acid increases. The equilibrium lies to the left because t-butanol is the weaker acid compared to water. You can also think this way: hydroxide is the weaker base (more stable) compared to t-butoxide because of e- donation of alkyl groups, making the resultant anion less stable (more reactive).

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Correct Answer: D. The rate order.

“SN” stands for nucleophilic substitution and the “1″ represents the fact that the rate-determining step is unimolecular.

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Correct Answer: C. 120^o

An sp²-hybridized carbon has a trigonal planar geometry. This means that the bond angle will be 120^o, so answer choice C is the best choice. An sp-hybridized carbon has a linear geometry, meaning a bond angle of 180^o. Finally, an sp³-hybridized carbon has a tetrahedral geometry with bond angles of 109.5^o.

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Correct Answer: C. N2(g) 0.75 and H2(g) 2.25 mol.L^-1.s^-1

The rate of consumption of the reactants will be the product of the rate of formation of the product multiplied by the ratio of the (co-efficient of the reactant) / (co-efficient of the product).

Rate N2(g) = 1.50 x 1/2 = 0.75 mole.L^-1.s^-1 and H2(g) 1.50 x 3/2 = 2.25 mole.L^-1.s^-1.

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Correct Answer: B. Adding a catalyst.

The rate at which a chemical reaction occurs is increased by increasing the temperature, adding a catalyst, increasing the concentration of a reactant in solution, increasing the partial pressure of a reactant gas and increasing the surface area of a reactant solid.

Catalysts increase the rate of reaction by effectively lowering the Activation Energy of the reaction, thus making the reaction easy to occur under any given conditions.

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