# SAT Subject Physics Practice Question 133: Answer and Explanation

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**Question: 133**

**6.** Two large glass containers of equal volume each hold 1 mole of gas. Container 1 is filled with hydrogen gas (2 g/mol), and Container 2 holds helium (4 g/mol). If the pressure of the gas in Container 1 equals the pressure of the gas in Container 2, which of the following is true?

A. The temperature of the gas in Container 1 is lower than the temperature of the gas in Container 2.

B. The temperature of the gas in Container 1 is greater than the temperature of the gas in Container 2.

C. The value of *R* for the gas in Container 1 is the value of *R* for the gas in Container 2.

D. The rms speed of the gas molecules in Container 1 is lower than the rms speed of the gas molecules in Container 2.

E. The rms speed of the gas molecules in Container 1 is greater than the rms speed of the gas molecules in Container 2.

**Correct Answer:** E

**Explanation:**

Neither A nor B can be correct. Using *PV* = *nRT*, both containers have the same *V*, *n* is the same, *P* is the same, and *R* is a universal constant. Therefore, *T* must be the same for both samples. C is also wrong, since *R* is a universal constant. The kinetic theory of gases predicts that the rms speed of the gas molecules in a sample of molar mass *M* and temperature *T* is

Hydrogen has a smaller molar mass than does helium, so *v*_{rms} for hydrogen must be greater than *v*_{rms} for helium (because both samples are at the same *T*).