How many grams of water are made from the reaction of 16.0 grams of oxygen gas?

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)