A #15*g# mass of methane is combusted with a #50*g# mass of dioxygen gas. Will combustion be complete?
You have the balanced chemical equation. Not only does this tell you the mass equivalence, i.e. how many grams of methane react with how many grams of dioxygen, you can include parameters that tell you how energy is transferred in the the reaction.
Here, there are
And for complete combustion, this requires
You thus have an insufficient mass of dioxygen for complete combustion. Complete combustion requires the given mass.
And so we don't know how much carbon dioxide will result; certainly it will be much less than the stoichiometric amount.