Mass of one mole (Avogadro Number) of neutrons ~ 1 g
Mass of the one neutrons =(1 / 𝐴𝑣𝑜𝑔𝑎𝑑𝑟𝑜 𝑁𝑢𝑚𝑏𝑒𝑟 (𝑁𝐴)) g
Mass of 8 molecule of water = 𝑀𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 / 𝑁𝐴
= 18 / 𝑁𝐴 g
There are 8 neutrons of water = 18 / 𝑁𝐴
Mass of one molecule of water = 𝑀𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 / 𝑁𝐴
= 18 / 𝑁𝐴 g
Fraction off mass of the water due to neutrons ~ 8 / 18
.