Methane and oxygen react to form carbon dioxide and water. what mass of water is formed if 3.2 g of methane reacts with 12.8 g of oxygen to produce 8.8 g of carbon dioxide?

The chemical equation for the combustion reaction of methane and oxygen is,
                            CH₄ + 2O₂ --> CO₂ + 2H₂O

Note that the equation is already balanced which means that both sides will have the same number of atoms of each elements. 

We use dimensional analysis and conversion to answer the question.
 
(1) 3.2 g methane
                  (3.2 g CH4)(1 mol CH4/16 g CH4)(2 mol H2O/1 mol CH4)(18 g H2O/1 mol H2O)
                   = 7.2 grams of H2O
(2) 12.8 g of oxygen
                  (12.8 g O2)(1 mol O2/16 g O2)(2 mol H2O/2 mol O2)(18 g H2O/1 mol H2O)
                          = 14.4 grams of H2O

(3) 8.8 g of carbon dioxide
       (8.8 g CO2)(1 mol CO2/44 g CO2)(2 mol H2O/1 mol CO2)(18 g H2O/1 mol H2O)
                       = 7.2 grams of H2O

Since, the least number among the three answers is 7.2 grams. The answer is therefore, 7.2 grams of water. 

The answer to this is the 3rd choice!