MCAT Question of the Day

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: A. PCC/CH2Cl2.

Pyridinium chlorochromate (PCC) is a mild oxidant, which converts primary and secondary alcohols to aldehydes and ketones respectively. KMnO4 and K2Cr2O7 are harsher oxidants, and would further oxidize primary alcohols to the corresponding carboxylic acids. Di-isobutylaluminum hydride (DIBAL) is a reducing agents.

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Correct Answer: C. Stratum corneum

If a layer of epidermis has a low rate of mitosis, this indicates that the cells in this layer are not dividing very frequently. Likewise the presence of keratin, signals that the layer of epidermis must have a waterproofing function. The stratum basale is the most mitotic layer and is the bottom layer of the epidermis. As the cells are produced in the stratum basale, older cells are pushed up to the stratum spinosum, which is also high in mitotic activity. Keratin dominates in the stratum corneum, which is the most superficial layer of the epidermis, characterized by dead skin cells that are no longer undergoing mitosis. Therefore, the correct answer is [C].

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Correct Answer: D. Cl2, FeCl3

This is an electrophilic ring transformation; it proceeds efficiently via electrophilic chlorination (“Cl+”) of the aromatic ring, which usually involves chorine plus a suitable Lewis acid (aluminum chloride).

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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. 10 m.

All of the kinetic energy will be transferred into potential energy at the maximum height of the cannonball. The trick is to not replace KE with (1/2)mv², the kinetic energy is already given! Therefore:

KE -> PE -> KE = PE -> KE = mgh -> h = KE/mg = 500 J / (5 kg)(10 N/kg) = 10 m.

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Correct Answer: D. The previous noble gas.

A metal prefers to lose electrons so that it has the configuration of 8 around its valence shell so that it is equivalent to the stable noble gas previous to it.

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Correct Answer: B. The cell will shrivel

For cells with a selectively permeable membrane, water will move from higher water concentrations to lower water concentrations. In this problem, the salt is too large to cross the membrane, so the salt concentration will remain the same. Since red blood cells have more water than the salt-water mixture, the water in the cell would leave to try to make the two environments isotonic. If water leaves the cell, the cell will eventually shrink.

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Correct Answer: A. 10^-5

Therefore:

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Correct Answer: B. Synapomorphies

Plesiomorphies are characteristics that are original in the organism. While apomorhic means “derived characteristic”, the prefix syn- alters the meaning to “shared derived characteristic.” Using the prefix aut- causes the meaning to change to “self-derived characteristic” implying that it isn’t shared by another group. Homoplasy or homoplastic characteristics are shared among groups, but weren’t derived from an ancestral source.

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