Dale P. answered • 02/04/21
Ph. D in Chemistry with 4 years of undergrad tutoring experience
The tricky thing here is to balance the equation. Balancing a combustion equation is never simple. First we need to write the chemical equation. The fact that the question says "completely" combusts is important as that means the product is CO2 rather than CO.
So the equation before being balanced is:
CH4 + O2 --> H2O + CO2.
From the get go we have two elements unbalanced. Oxygen has 2 on the left and three on the right, and hydrogen has four on the left and 2 on the right. Let's start with hydrogen as the math is easier. I we double water we'll match 4 for 4.
CH4 + O2 --> 2 H2O + CO2.
Okay we solved our hydrogen problem, now what about oxygen? At first glance it would seem we made it worse since now we have 2 on the left and 4 on the right. But that's actually perfect! Oxygen gas is a dimer so if we add a second molecule of that everything should balance perfectly.
CH4 + 2 O2 --> 2 H2O + CO2
Okay we have our equation. Now we need to find out how many moles of methane we have. We have 14.7 g of methane and the molecular mass of methane is 16.043 g/mol. All we need to do is divide the mass by the molar mass and then we have the amount of methane in moles. From the balanced equation we know we need twice as many moles of O2 as moles of CH4 ( 1 CH4 + 2 O2). Just don't forget that you have 3 sig figs!
