Question: how many moles of water would be produced if 5 moles of propane are combusted?
Answer: Use stoichiometry to calculate how many moles of water can be produced from the complete combustion of 5 moles of propane. Multiply 5 moles C3H8 by the mole ratio between C3H8 and H2O so that moles C3H8 cancel, leaving moles H2O. The complete combustion of 5 moles of C3H8 will produce 4 moles of H2O.