Example Question #7 : Stoichiometry
Suppose that 8 grams of of hydrogen gas and 16 grams of oxygen gas are ignited. What is the theoretical yield, in grams, of water? Assume hydrogen has an atomic mass of 1 and oxygen has an atomic mass of 16.
First, convert from grams to moles using the molar masses of the compounds. There are 4 mol of hydrogen gas and 0.5 mol of oxygen gas. Oxygen gas is clearly the limiting reagent (since there is more than twice as much hydrogen gas as oxygen gas), which means that the theoretical yield of water is
Water contains two hydrogen atoms and an oxygen atom, so it has a molar mass of
Hence, 1 mol of water would have a mass of 18 g.