A mixture of $CH X_{4}$ ( $16 g/mol$ ) and $O X_{2}$ ( $32 g/mol$ ) has a density of $1.340 g/L$ at STP (molar volume $22.4 L/mol$ ). Find the volume percent of $CH X_{4}$ in the mixture, to one decimal place.

Numerical answer type, answer in %, tolerance 0.1.

Answer: 12.4 %

Average molar mass = density x molar volume = 1.340 x 22.4 = 30.016 g/mol. With mole fraction x of

CH X_{4}

: 16x + 32(1-x) = 30.016, so x = (32 - 30.016)/16 = 0.1240. Volume % equals mole %, i.e. 12.4%.