Question: how many grams of h2o are produced when 2.5 moles of oxygen are used?
Answer: Thus, when 2.5 mol of oxygen reacted with hydrogen, 2 * 2.5 mol = 5 mol of water is produced. The molar mass of water is 18 g/mol. One mol of water weighs 18 g. 5 mol of water weights 5 * 18 = 90 g.