- 8/18

Mass of one mole (Avogadro Number) of neutrons - 1 g

Mass of one neutron = 1 / Avogadro Number (N_{A})g

Mass of one molecule of water = Molar mass / N_{A } = 18 /N_{A} g

There are 8 neutrons in one atom of oxygen

Mass of 8 neutrons = 8/ N_{A}

Fraction of mass of water due to neutrons - 8 /18