Posted by sim on .
a 5 g mixture of natural gas containing ch4 and c2h4 was burnt in excess of oxygen yielding 14.5 g of co2 and h2o.What is the weight of ch4 and co2 in the mixture

chemistry 
DrBob222,
CH4 + 2O2 ==> CO2 + 2H2O
C2H4 + 3O2 ==> 2CO2 + 2H2O
Let x = mass CH4
and y = mass C2H4

You need two equations.
(eqn 1) is total mass = 5g
(eqn 2) is (mass CO2+H2O from CH4) + (mass CO2+H2O from C2H4) = 14.5g

Now convert those into x and y and solve. I have let mm stand for molar mass below.
(eqn 1) is x + y = 5
(eqn 2) is
[x(mmCO2/mmCH4)+x(2*mmH2O/mmCH4)]+y(2*mmCO2/mmC2H4)+y(2mmH2O)/mmC2H4] = 14.5g
Solve for x = mass CH4
y = mass C2H4.
Post your work if you get stuck.