# Hydrocarbon empirical formula question

1. Oct 14, 2009

### lastlanding

1. The problem statement, all variables and given/known data
Q. A Hydrocarbon (CnHm) 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

2. Relevant equations

pV=nRT
pa/ptotal = na/ntotal

3. The attempt at a solution
The answer is C3H8 aka Propane

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

2. Oct 14, 2009

### 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.