Percentage (by mass) of elements given in the body of a healthy human adult is : Oxygen =61.4%, Carbon =22.9% Hydrogen =10.0% and Nitrogen =2.6% ∵ Total weight of person =75kg ∴ Mass due to 1H is =75×
10
100
=7.5kg If 1H atoms are replaced by 2H atoms. Mass gain by person would be =7.5kg