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 `.^(1)H` atoms are replaced by `.^(2)H` atoms is:

Options

A. 10 kg
B. 15 kg
C. 37.5 kg
D. 7.5 kg

Explanation

To solve the problem of how much weight a 75 kg person would gain if all the \( ^{1}H \) atoms in their body were replaced by \( ^{2}H \) atoms, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the mass percentage of hydrogen in the human body**:
The problem states that hydrogen constitutes 10% of the body mass.

2. **Calculate the mass of hydrogen in a 75 kg person**:
\[ \text{Mass of hydrogen} = \text{Total mass} \times \left(\frac{\text{Percentage of hydrogen}}{100}\right) = 75 \, \text{kg} \times \left(\frac{10}{100}\right) = 7.5 \, \text{kg} \]

3. **Understand the replacement of hydrogen atoms**:
When \( ^{1}H \) (protium) is replaced by \( ^{2}H \) (deuterium), the mass of hydrogen effectively doubles because \( ^{2}H \) is approximately twice as heavy as \( ^{1}H \).

4. **Calculate the new mass of hydrogen after replacement**:
\[ \text{New mass of hydrogen} = \text{Original mass of hydrogen} \times 2 = 7.5 \, \text{kg} \times 2 = 15 \, \text{kg} \]

5. **Calculate the weight gain**:
The weight gain is the difference between the new mass of hydrogen and the original mass of hydrogen:
\[ \text{Weight gain} = \text{New mass of hydrogen} - \text{Original mass of hydrogen} = 15 \, \text{kg} - 7.5 \, \text{kg} = 7.5 \, \text{kg} \]

### Final Answer:
The weight that a 75 kg person would gain if all \( ^{1}H \) atoms are replaced by \( ^{2}H \) atoms is **7.5 kg**.

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