MCAT General Chemistry Question 119: Answer and Explanation

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Question: 119

14. In which of the following situations is it impossible to predict how the pressure will change for a gas sample?

  • A. The gas is cooled at a constant volume.
  • B. The gas is heated at a constant volume.
  • C. The gas is heated, and the volume is simultaneously increased.
  • D. The gas is cooled, and the volume is simultaneously increased.

Correct Answer: C

Explanation:

Both a change in temperature and a change in volume can affect a gas's pressure. So if one of those two variables is kept constant, as in choices (A) and (B), we'll definitely be able to predict which way the pressure will change. At a constant volume, heating the gas will increase its pressure, and cooling the gas will decrease it. What about when both temperature and volume are changing? If both changes have the same effect on pressure, then we can still predict which way it will change. This is the case in choice (D). Cooling the gas and increasing its volume both decrease pressure. Choice (C), on the other hand, presents too vague a scenario for us to predict definitively the change in pressure. Heating the gas would amplify the pressure, while increasing the volume would decrease it. Without knowing the magnitude of each influence, it's impossible to say whether the pressure would increase, decrease, or stay the same.

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