Atomic Number. It represents the number of protons in an atom of a particular element, which is what identifies that particular element. If the number of protons is different, the element is different.
M of water = 2(1) + 16 = 18 g/mol
n of water = 24g/(18g/mol) = 4/3 mol
n of H2O : n of O2 = 2 : 1
2:1 = 4/3 : x
x = (4/3)/2 = 2/3 mol of O2
48 cal is what I put on mine