# Chemistry

posted by Michelle

A 10.0 gram sample of a mixture of CH4 and C2H4 reacts with oxygen at 25°C and 1 atm to product carbon dioxide gas and liquid water. If the reaction produces 520 kJ of heat, what is the mass percentage of CH4 in the mixture?

1. DrBob222

This is a long one and will take your complete concentration to follow but here is what you do. Here are the combustion equations.
rxn 1. CH4 + 2O2 ==> CO2 + 2H2O
rxn 2. C2H4 + 3O2 ==> 2CO2 + 2H2O
Next you calculate the heat evolved with rxn 1 and rxn 2. These are done by looking up the dHformation in tables in your text/notes. I will call these dH1 and dH2.
dH1 = (n*dHf products) - (n*dHf reactants)
dH2 = (n*dHf products) - (n*dHf reactants)
You will get a number for dH1 and dH2.
You use this information to set up and solve two equations simultaneously. You will need the two equations which I will call eqn 1 and eqn 2.

Let X = mass CH4
and Y = mass C2H4
---------------------
eqn 1 is X + Y = 10 grams.

eqn 2 is made up of the heat generated by rxn 1 + heat generated by rxn 2 to produce 520 kJ. I will let mm stand for molar mass. The heat generated by CH4 will be (X/mm CH4)*dH1.
The heat generated by C2H4 will be
(2Y/mm C2H4)*dH2 and you put these together to make eqn 2 (which is to be solved with eqn 1).
(X/mm CH4)*dH1 + (2Y/mm C2H4)*dH2 = 520

Solve eqn 1 and eqn 2 for X and Y then plug into the % formula to find % CH4 and %C2H4

%CH4 = (X/10)*100 = ?
%C2H4 = (Y/10)*100 = ?

2. Michelle

What does n equal in the dH equations? number of moles?

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