MCAT General Chemistry Question 212: Answer and Explanation

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Question: 212

4. There are an unknown number of moles of argon in a steel container. A chemist injects two moles of nitrogen into the container. The temperature and volume do not change, but the pressure increases by ten percent. Originally the container held:

  • A. 16 moles of Ar.
  • B. 18 moles of Ar.
  • C. 20 moles of Ar.
  • D. 22 moles of Ar.

Correct Answer: C

Explanation:

C At constant V and T, the pressure of an ideal gas reflects the number of particles (regardless of their identity). It is a simplification of the ideal gas law from PV = nRT to P ∝ n. So, if the addition of two moles of N2 into the chamber results in an increase in P of 10 percent, then the moles added must be 10 percent of the initial number of Ar moles. Two moles are 10 percent of 20 moles.

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