Hydrocarbon empirical formula question

[lastlanding]

Homework Statement

Q. A Hydrocarbon (C_nH_m) is burned completely in oxygen to produce a mixture of CO₂ and H₂O. The total pressure of mixture is 1.200 atm and partial pressure of H₂O is 0.686 atm. What is empirical formula of Hydrocarbon

Homework Equations

pV=nRT
p_a/p_total = n_a/n_total

The Attempt at a Solution

The answer is C₃H₈ aka Propane

Im so confused! I don't even know where to start with this!

 

[Delphi51]

Find the partial pressure for the CO2. The partial pressures of the gases are proportional to the number of molecules, so you've immediately got the ratio of the number of H2O molecules to the number of CO2 molecules. This is almost m/n, but note that because of the H2 there are twice as many H's as H2O's, so you've really got the ratio of m/2 to n. It should then be easy to get the ratio m to n. Then you start guessing - if n=1 does m work out to an even number? If not, try n = 2 and so on.