We need a balanced equation first:
4NH3 + 7O2 --> 6H2O + 4NO2
First, convert from molecules of NH3 to moles of NH3:
(2.1x10²⁴ molecules)/(6.02x10²³)= 3.49mol NH3
Second, using the balanced equation, convert moles of NH3 to moles of H2O:
(3.48mol NH3)(6mol H2O/4mol NH3) 5.22mol H2O
Third, convert moles of H2O to grams:
(5.22mol H2O)(18.02g/mol H2O) = 94.1g H2O
