MCAT Question of the Day

Correct Answer: D. The pH will remain close to 5.

Buffers are designed to maintain pH at a certain level, and operate through the complementation of a conjugate acid and base. As long as the ratios of acid and base remain the same in the buffer system, and they will no matter what the dilution, as long as the dilution is not very very small (i.e. 1 mM or less), the pH will remain the same.

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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. 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: 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: D. Hydrochloric acid pH = 0; acetic acid pH = 4; water pH = 7; ammonia solution pH = 10; sodium hydroxide solution pH = 14.

Acids have pH values of less than 7, with the stronger the acid the lower the pH. Bases have pH values of greater than 7, with the stronger the base the higher the pH. Typically:

• Strong acids have a pH of 0 or less, hydrochloric acid pH = 0.
• Weak acids between pH 3-6, acetic acid pH = 4.
• Neutral solutions have pH = 7, water pH = 7.
• Weak bases have a pH 8-10, ammonia solution pH = 10.
• Strong bases have a pH of 14 or more, sodium hydroxide solution pH = 14.

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Correct Answer: D. 3.17 x 22.4 = 71.0

STP is Standard Temperature and Pressure or 0^oC and 1 atm pressure. At STP 1 mole of an ideal gas occupies 22.4 L. A good approximation of the molecular mass of a gas is obtained by multiplying its density in g.L^-1 by 22.4 L to attain the mass of 1 mole of gas. Hence: 3.17 x 22.4 = 71.0

24.5 L is the molar volume of an ideal gas under LTP or Laboratory Temperature and Pressure, a balmy 25^oC and 1 atm pressure.

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Correct Answer: C. 6000 N-m

Assuming that the water stream is tangent to the wheel, it will be perpendicular to the radius vector at the point of contact. Therefore, the torque of the force of the water stream is Torque = r*F = (20m)(300 N) = 6000 N-m.

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Correct Answer: A. The method relies on the fact that solids are more soluble in a cold solvent than they are in the same amount of hot solvent.

This statement is reversed: The process of recrystallization involves dissolution of the solid in appropriate solvent at a high temperature and the subsequent re-formation of the crystals upon cooling, so that any impurities remain in solution.

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Correct Answer: D. Boiling and condensation

Melting: solid to liquid and Freezing: liquid to solid occur at the Melting/Freezing Point.
Boiling: liquid to gas and Condensation: gas to liquid occur at the Boiling/Condensation Point.
Both evaporation: liquid to gas at the surface of the liquid and sublimation: solid to gas occur at any temperature.

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Correct Answer: D. pH of the titrant

During a titration procedure, a buret is lowered into a clean Erlenmeyer flask. The initial volume of titrant is recorded and a stream of titrant is delivered while constantly mixing the sample solution. Once the pH indicator changes colors we have reached the endpoint. The final volume is recorded and used in conjunction with the initial volume to find the volume of titrant used, and along with the concentration of the titrant to find how many moles of titrant were used. Finally, we can use stoichiometry of the titration reaction to calculate the number of moles of reactant in our solution. The pH of the titrant is not needed.

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