How much oxygen will burn in the following combustion reaction?

Question

In a particular reaction, 4.5 moles of heptane burn in the presence of oxygen. C7H16 + 11O2 -> 7CO2 + 8H2O How many moles of oxygen will burn?

 

I'd like to understand HOW to arrive at the answer, not the answer by itself please.  My text does not cover how to work with this type of problem, instead has it listed as a "self study"...very frustrated.

Answer 1

The balanced chemical equation can be read as:

 

one mole of heptane combines with 11 moles of molecular oxygen to produce 7 moles of carbon dioxide plus 8 moles of water.

 

In particular, 1 mole of heptane will require 11 moles of molecular oxygen to be fully consumed in the burn.  So  4.5 moles of heptane will require  4.5 x 11 = 49.5 moles of molecular oxygen.  

 

This type of problem is called a mole-mole problem.  Mole-mole problems can be solved by scaling up the chemical equation as I did for this case.    More difficult problems involve conversion of mass to moles.    Remember:  the chemical equation can be read in moles.