Question

What is a mass fraction of neutrons in a water molecule?

• A. 4/9
• B. 1/3
• C. 8/9
• D. 2/3

Answer 1

The correct option is A 4/9

Mass of one neutron is $1$ amu.
Mass of one water molecule is $18$ amu.
Oxygen atom contains $8$ neutrons and hydrogen atom contains no neutrons.

So the mass of neutrons in one water molecule is $8amu$.
Fraction of mass of water due to neutrons is $\frac{8}{18}=\frac{4}{9}$