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
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