Physics quiz question: moles of ideal gas

Physics 5A Quiz 7, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 13.70

[3.0 points.] There are two identical volume tanks of gas at 15.0° C and at atmospheric pressure, one containing H2 molecules and the other He atoms. The atomic masses of hydrogen and of helium are 1.00 u and 4.00 u, respectively. Which tank contains more moles of gas?
(A) The H2 tank.
(B) The He tank.
(C) (The H2 tank and the He tank each contain the same moles of gas.)
(D) (Not enough information is given to determine this.)

Correct answer: (C)

From the macroscopic form of the ideal gas law:

P*V = n*R*T,

and as each tank is at the same pressure, has the same volume, and is at the same temperature, and thus must contain the same number of moles of gas.

Student responses
Sections 0906, 0907
(A) : 17 students
(B) : 12 students
(C) : 5 students
(D) : 0 students