Ethane gas reacts with 670 grams of oxygen gas forming carbon dioxide gas and liquid water. How many grams of water are produced?

2 C2H6 + 7 O2 = 4 CO2 + 6 H2O

we are not given the mass of ethane so we assume it is in exces and O2 is the limiting reagent

moles of O2 in the reaction = 670 / 32 = 20.94 moles

the molar ratio of O2 to H2O is 7:6 so moles of H2O produced = 20.94 * 6/7 = 17.95 moles

mass of H2O produced = 17.95 moles * 18 g / mol = 323 g