Question

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 kg person would gain if all $^{\text{1}}\text{H}$ are replaced by $^{\text{2}}\text{H}$atoms is
a.) 15 kg
b.) 37.5 kg
c.) 7.5 kg
d.) 10 kg

Answer 1

Hint: Solve this question by calculating the mass of hydrogen using the given percentage. Also note that $^{\text{1}}\text{H}$ and $^{\text{2}}\text{H}$ are isotopes of hydrogen.

Complete step by step answer:
According to the question,
Weight of the person = 75 kg
Mass % of Oxygen = 61.4 %
Mass % of Carbon = 22.9 %
Mass % of Hydrogen = 10 %
Mass % of Nitrogen = 2.6 %
Therefore, mass of $^{\text{1}}\text{H}$= 10 % of 75 kg = 7.5 kg
Since $^{\text{2}}\text{H}$ is double the mass of $^{\text{1}}\text{H}$,
Therefore, mass of $^{\text{2}}\text{H}$= 2 x 7.5 kg = 15 kg
According to the question, all$^{\text{1}}\text{H}$ are replaced by $^{\text{2}}\text{H}$.
So, the increase in weight = weight of $^{\text{1}}\text{H}$ – weight of $^{\text{2}}\text{H}$= 15 – 7.5 kg = 7.5 kg
Therefore, the answer is – option (c) – If all $^{\text{1}}\text{H}$ are replaced by $^{\text{2}}\text{H}$, the weight gained by the person is 7.5 kg.

Note: “Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom”.
There are three isotopes of hydrogen, namely - protium ($^{\text{1}}\text{H}$), deuterium ($^{\text{2}}\text{H}$) and tritium ($^{\text{3}}\text{H}$).