30cm^3 of a mixture of methane and ethyne require 70cm^3 of oxygen for complete combustion. Calculate the composition of the mixture

Answer 1

This is a problem with two unknowns (at least the way I'm solving it) so it requires two equations and you solve them simultaneously.
Write and balance the equations.
CH4 + 2O2 ==> CO2 + 2H2O
2C2H2 + 5O2 ==> 4CO2 + 2H2O

Let X = cc of CH4
and Y = cc of C2H2
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equation 1 is X + Y = 30 cc
equation 2 is cc oxygen in tems of X and Y
(2X) is cc O2 in terms of X for combustion of CH4 and
(5Y/2) is cc O2 in terms of Y for combustion of C2H2. Put those together to get equation 2 which is
2X + 5Y/2 = 70 cc..........equation 2
X + Y = 30 cc ..............equation 1
Solve for X and Y.
The problem doesn't say how it wants the composition; usually this is done as a percent so
(X/70)*100 = %CH4
(Y/70)*100 = %C2H2
Post your work if you get stuck.