MCAT Question of the Day

Correct Answer: D. None of the above.

Only temperature will change the Ka, Kb, Kw, Keq, and Ksp. The Ksp will be constant and not change under the conditions provided in the answer choices.

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Correct Answer: C. I and III only.

According to ideal gas assumptions, all gas molecules are very far away from other gas molecules and their volumes are negligible. Should two gas molecules collide, their collision will be elastic, meaning no kinetic energy is lost (the average kinetic energy can only be changed by heating/cooling the gas). While it is true pressure is caused by the collision of gas molecules against the walls of the container, this is not an assumption for an ideal gas, but rather a general fact of nature. We typically want our ideal gases at low pressures too.

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Correct Answer: B. 0.1

Use P1V1 = P2V2. Temperature is constant and can be eliminated from the equation. If the volume increases, we know the pressure must decrease. Doubling the volume should then halve the pressure.

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Correct Answer: D. (746 – 24)/760

You must subtract the pressure of water vapor from the total pressure, and change torr to atm. 1 atm = 760 torr = 760 mm Hg.

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Correct Answer: D. (1.00)(0.20)/(0.082)(373)

Use n = PV/RT. Remember R is a constant and that all the units must be converted into atm of pressure, liters of volume, and Kelvin from Celsius.

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Correct Answer: A. C-1

Glucose anomers have a different orientation of the hydroxyl group on the C-1 carbon.

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Correct Answer: A. I only.

At low densities the correction terms to P and V become very small. Both correction terms, nb and an²/V², depend on the number of gas molecules in a given volume, which is small at low densities. PV is not constant for the real gas as it is for an ideal gas.

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Correct Answer: D. Ar < Cl2 < FBr

The order of boiling points corresponds to the strength of intermolecular forces. Argon has the weakest intermolecular forces: as a neutral atom, it is less polarizable than the larger Cl2. The latter has no dipole moment, so it exerts weaker forces than FBr.

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Correct Answer: D. II and III only

Ideal gases do indeed move faster when heated due to the increase in average kinetic energy. However, we assume that there are no intermolecular forces between molecules, therefore they are infinitely compressible in theory. We also assume that an ideal gas will not change into a liquid no matter the conditions.

Correct Answer: B. -OH is a poor leaving group and therefore must be protonated to form a better leaving group, -OH2+.

The hydroxide group is a strong base and therefore a poor leaving group. It must be protonated to form the weak base, H2O, so that it can leave the molecule.