Find the percent by mass of oxygen in H2O.

Use atomic masses: H = 1.008 g/mol and O = 16.00 g/mol.
molar mass H2O = 2(1.008) + 16.00 = 18.016 g/mol
percent oxygen = (16.00 / 18.016) * 100%
percent oxygen = 88.8% oxygen.