Chemistry  Quantitative analysis
posted by RJ .
A mixture weighing 7.290 mg contained only cyclohexane, C6H12 (FM 84.159), and oxirane, C2H4O (FM 44.053). When the mixture was analyzed by combustion analysis, 21.999 mg of CO2 (FM 44.010) were produced. Find the weight percent of oxirane in the mixture.

The combustion equations are as follows:
C6H12 + 9O2 ==> 6CO2 + 6H2O
2C2H4O + 5O2 ==> 4CO2 + 4H2O
This is an equation with two unknowns. Here are the two equations which you should solve simultaneously.
Let Y = milligrams cyclohexane
and Z = milligrams oxirane
===================
Y + Z = 7.290 is eqn 1.
(6Y*44.01/84.159) + (2Z*44.01/44.053) = 21.999 is eqn 2.
After you find Z, then
%Z = (Z(in mg)/7.290)*100 = ? 
Thank you DrBob222!

10.5 wt%