What mass of water is produced when 2.1 x 10^24 molecules of ammonia (nh3) react with sufficient oxygen? nh3 + o2 → h2o + no2

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