In the explosion of a hydrogen-filled balloon, 0.60 g of hydrogen reacted with 4.8 g of oxygen. how many grams of water vapor are formed? (water vapor is the only product.)

First write the balanced equation of this reaction:

2H2 + O2 - > 2H2O

mol of H2 = 0.60 gH2/2.02 gH2 = 0.297 mol

There are 2 mol of H2 for every 2 mol of H2O so the number of mol of H2 is equal to the number of mol of H2O.

g of H2O = 0.297 mol H2O • 18.02 gH2O = 5.35 g H2O

Do the same thing for O2:

mol of O2 = 4.8 gO2/32.0 gO2 = 0.15 mol of O2

There is 1 mol of O2 for every 2 mol of H2O so multiply 0.15 • 2 to get the number of mol of H2O

g of H2O = 0.30 mol H2O • 18.02 gH2O = 5.41 g H2O

The correct answer is 5.35 g H2O (or 5.4 g if checking significant figures) because O2, in this case, is the limiting reactant of this reaction.