Question

The most abundant elements by mass in the body of a healthy human adult is oxygen,nitrogen,carbon,hydrogen .If the hydrogen is replaced by 2 hydrogen then how is the mass of the body affected

Answer 1

If it is replaced by  then mass will be doubled so now hydrogen mass = 15 kg So, mass of person will be increased by 7.5 kg.

or

Since H is 10% and 1H atoms are replaced by 2H atoms then weight of hydrogen is just doubled. Weight gain will be 75 x .1 = 7.5 kg