MCAT Question of the Day

Correct Answer: D. 30N.

Since the object is not moving, its acceleration must be zero, so the net force on it must also be zero. Therefore, the magnitude of the buoyant force upward must equal the magnitude of the weight downward, the specific gravity is extraneous information:

B = w = mg = (3 kg)(10 N/kg) = 30 N

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Correct Answer: C. Vacuole

The correct answer is C. Cells within the Kingdom Plantae have chlorophyll containing cells called CHLOROPLASTS, a rigid cell WALL outside of the plasma membrane, and vacuoles that serve to store carbohydrates. Plant cells do not contain lysosomes, as other eukaryotic cells, instead they have peroxisomes for digestion of foreign material.

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Correct Answer: C. The k value would increase, but only slightly.

The correct answer is C. The Arrhenius equation is given by k = Ae^-E/RT, where A is the pre-exponential factor, R is the gas constant, T is the temperature, and E is the activation energy. Thus, if the temperature were increased by a factor of 4, then the exponent would decrease by a factor of 4, since T is in the denominator. However, since it is e^-x, the actual value won’t change much. For example, exp(-1) = 0.36, but exp(-1/4) = 0.77. Thus, although the value of k will increase, it will not increase by a factor of 4, only slightly.

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Correct Answer: C. 3

Shown here is a Wittig reaction, or Wittig Olefination, which converts aldehydes to alkenes through reaction with a phosphonium ylide.

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Correct Answer: B. Polarimeter.

A polarimeter is a device used for measuring the angle of rotation of plane-polarized light passed through optically active substances. When passed through these substances, optically active substances will cause the polarized light to rotate to either the left (counter-clockwise). The other instruments listed are unable to distinguish between enantiomeric forms.

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Correct Answer: B. An ionic bond.

An ionic bond is one in which both parts of the bond are highly positive or negative so that they are attracted strongly to each other.

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Correct Answer: A. 3-Ethyl-3,4-dimethylpentanoic acid

1. First, identify the (longest) parent chain -> 5 carbons; the suffix used is “pent-“.
2. Recognize that the carboxylic acid, being the only functional group, takes highest priority and is the parent functional group -> begin numbering the chain here.
3. Side chains: ethyl has locant = 3, methyl has locant = 3, 4.
4. Prefixes di-, tri- etc are NOT considered when placing substituents in alpha order.

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Correct Answer: B. 2

Carbon-centered radicals follow the same rules of stability as carbocations: the greater number of alkyl groups directly connected to a carbon atom, the greater the stabilization. Of the choices, B and D have the greater substitution around the radical carbon center (2^o). B, however, can also be described as an allylic radical; the pi system of a double bond stabilizes an adjacent radical through resonance. Remember that delocalizing the radical is a stabilizing effect.

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Correct Answer: B. Polyphyletic and heterotrophic.

Fungi are a classified as a group based upon common biological traits acquired through convergent evolution, without a common ancestor, and are therefore considered polyphyletic. Unlike plants, fungi cannot make their own food supply. Instead, they obtain nutrition by parasitism, saprobism or symbiosis.

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Correct Answer: B. The shark and the dolphin

Convergent evolution is the process where organisms from different ancestral linage, grow to look alike and share similar physical characteristics because they live in the same or very similar environments. Whereas divergent evolution is defined as the evolutionary process whereby organisms from the same ancestral lineage differ in appearance. This difference would result because of the different habitats of the organisms. Only the shark and the fish are from different animal lineages and look very similar because they both live in aquatic environments. The other animal pairings all originate from a similar ancestral lineage and have grown to look very different over time because of their terrestrial habitats.

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