MCAT Question of the Day

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: 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: 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: A. Speed = 6.67 m/s and Velocity = zero

Speed is a scalar quantity involving magnitude only: Speed = distance/time = 400/60 = 6.67 m/s. Velocity is a vector quantity involving both magnitude and direction: Velocity = displacement/time = 0/60 = zero.

Distance traveled is not dependent on a direction, but displacement is. Displacement = distance from the finish to the start, which in this case is zero as the athlete has run around an oval to arrive back at her starting place.

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Correct Answer: C. cellular respiration

In the cell, metabolism is defined as the chemical reactions that occur within the cell to sustain life. Metabolism can further be divided into anabolism (the synthesis of new compounds) and catabolism (breaking down of substances). Answer options A, B, D, and E are all anabolic processes, that require the use of energy to make new compounds. In cellular respiration, glucose is broken down to produce ATP, therefore this is a catabolic process.

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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. 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 amount of salt reabsorbed in the kidneys will increase.

Aldosterone is a hormone released from the adrenal gland. Its presence in the blood helps the kidney’s retain more sodium chloride (NaCl) in the nephrons, thereby desalinating the urine. Glucose regulation is controlled by the pancreas’ release of insulin. Calcium ion levels are controlled by calcitonin and parathyroid hormone. The amount of water retained by the kidneys is controlled by the release of the antidiuretic hormone (ADH) or vasopressin. Aldosterone helps to retain water too, so the amount of water excreted by the body will decrease. Therefore, the correct answer is [D].

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Correct Answer: C. 16 W

When resistors are in parallel, we can find equivalent resistance simply by using the equation:

(1/Req) = (1/R1) + (1/R2) + … + (1/Rn)

In this case, the equivalent resistance is: (1/Req) = (1/12)+(1/12)+(1/12)= 3/12

We find that Req = 4. We then input this into our equation for the power of a circuit

P = IV = V²/R = 8²/4= 16W

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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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