Question:
What is the fraction of mass of water $\left(\mathrm{H}_{2} \mathrm{O}\right)$ due to neutrons?
Solution:
Molecular mass of $\mathrm{H}_{2} \mathrm{O}=2 \times 1+16=18 \mathrm{u}$
Total number of neutrons $=2 \times 0+8=8$
Mass of neutrons $=8 \mathrm{u}$
Fraction of mass due to neutrons $=\frac{\text { Mass of neutrons }}{\text { Molecular mass of } \mathrm{H}_{2} \mathrm{O}}=\frac{(8 \mathrm{u})}{(18 \mathrm{u})}$
$=\frac{8}{18}=\frac{4}{9}$ or $4: 9$