Assuming hydrogen gas is in excess, calculate the moles of oxygen
n(O2) = 16.0 g / 32.0 g/mol = 0.500 mol
Calculate the moles of water using the mole ratio; the ratio of H2O to O2 is 2:1 or the moles of H2O is twice the moles of O2
n(H2O) = 2 x n(O2) = (2)(0.500) = 1.00 mol
Calculate the mass of H2O by multiplying the moles by the molar mass of water
mass = (1.00 mol)(18.0 g/mol) = 18.0 g H2O
Ans: A)
