A mixture of hydrogen ( $M = 2$ ) and helium ( $M = 4$ ) has total mass 5.0 g and occupies 33.6 L at STP (22.4 L/mol). Find the mole percent of helium, to the nearest integer.

Numerical answer type, answer in %, tolerance 1.

Answer: 67 %

Total moles = 33.6/22.4 = 1.5. Let moles of

H X_{2}

= a: 2a + 4(1.5 - a) = 5, so a = 0.5 and He = 1.0 mol. Mole percent of He = 1.0/1.5 x 100 = 66.67, i.e. 67%.

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