The most abundant elements by mass in the body of a healthy human adult are :

The most abundant elements by mass in the body of a healthy human adult are : Oxygen $(61.4 \%)$; Carbon $(22.9 \%)$, Hydrogen $(10.0 \%)$; and Nitrogen $(2.6 \%)$. The weight which a $75 \mathrm{~kg}$ person would gain if all ${ }^{1} \mathrm{H}$ atoms are replaced by ${ }^{2} \mathrm{H}$ atoms is