The most abundant elements by mass in the body of a healthy human adult are : oxygen $$\left( {61.4\% } \right),$$  carbon $$\left( {22.9\% } \right),$$  hydrogen $$\left( {10.0\% } \right)$$  and nitrogen $$\left( {2.6\% } \right).$$  The weight which a $$75\,kg$$  person would gain if all $$^1H$$  $$atoms$$   are replaced by $$^2H$$  $$atoms$$   is

Solution :
Mass of elements in the body of a healthy human adult are :
Oxygen $$\left( {61.4\% } \right),$$  carbon$$\left( {22.9\% } \right),$$  hydrogen $$\left( {10\% } \right)$$  and nitrogen $$\left( {2.6\% } \right).$$
Weight of the person $$ = 75\,kg$$
Mass due to $$^1H = 75 \times \frac{{10}}{{100}} = 7.5\,kg$$
On replacing $$^1H$$  by $$^2H,7.5\,kg$$    mass would replace with $$15\,kg.$$
∴ Net mass gained by person $$ = \left( {15 - 7.5} \right)kg = 7.5\,kg$$