# MCAT Physics Question — Molar Mass of Gas

• Reviewed By: Liz Flagge
• An ideal gas at STP occupies 0.03L and has a mass of 0.002g. Which of the following gives the correct expression for the molar mass of the gas?

A) 22.4 x 0.03 / 0.002

B) 0.03 x 0.002 / 22.4

C) 22.4 x 6.02 x 1023 / 0.002

D) 22.4 x 0.002 / 0.03

Explanation

Molar mass is given in g/mol so we must simply re-arrange the units given to get g/mol. The MCAT also wants us to remember that at STP, one mole of gas occupies 22.4L.

Thus the right answer is (D):

22.4 L / 1 mol x 0.002g / 0.03L = 22.4 x 0.002 / 0.03 g/mol

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