Question

What is the fraction of the mass of water due to neutrons?

Answer 1

Mass of one mole (N_A) of neutrons = 1g

Mass of 1 mole of H₂O = 18

Mass of one molecule of water = `18/"N"_"A""g"`

Number of neutrons in one atom of H = 0

Number of neutrons in one atom of O = 8

Mass of 8 neutrons (in H₂O) = `8/"N"_"A"`

Fraction of mass of water due to neutrons 

= `(8/"N"_"A")/(18/"N"_"A")` or `8/18 = 4/9`