Solution.
The unbalanced reaction, showing only the reactants and products, is



since we know that the combustion products are carbon dioxide and water. The balancing process is given in the illustration opposite:
In brief, if three molecules are formed from the three carbon atoms in one propane molecule, then six oxygen atoms or three molecules will be required. In addition, the eight hydrogen atoms in propane will lead to four water molecules, thereby requiring two more molecules of . The balanced equation is



The molecular weights of reactants and products are

: 3 x 12.01 g + 8 x 1.01 g = 44.11 g
: 2 x 16.00 g = 32.00 g
: 1 x 12.01 g + 2 x 16.00 g = 44.01 g
: 2 x 1.01 g + 1 x 16.00 g = 18.02 g

From the balanced equation and these molecular weights we can verify that mass is conserved during the reaction: