# DBT JRF 2022 — Question 50

Exam: DBT JRF 2022 · Section: A · Topic: Biochemistry › Solution Chemistry & Molarity · Difficulty: Medium

## Question

What is the concentration of $D_2O$ in heavy water (density 1.11 g/ml)?

## Options

- **A.** 0.5 M
- **B.** 5.0 M
- **C.** 50.0 M
- **D.** 55.5 M

## Answer

Option D

## Explanation

Concentration of D2O (heavy water) given density = 1.11 g/mL: Molar mass of D2O: $M = 2(2.014) + 15.999 \approx 20.03 \approx 20\,\text{g/mol}$ Density = 1.11 g/mL = 1110 g/L Molarity: $C = \frac{\text{density}}{M} = \frac{1110\,\text{g/L}}{20\,\text{g/mol}} = 55.5\,\text{M}$ Note: This is analogous to calculating the molarity of pure water (H2O): $\frac{1000\,\text{g/L}}{18\,\text{g/mol}} = 55.6\,\text{M}$ . D2O is slightly denser and slightly heavier, giving a very similar (~55.5 M) answer. Answer: 55.5 M (Option 4).

Source: https://mypyqbuddy.com/questions/what-is-the-concentration-of-d-2o-in-6a104cd5920734408bc0c70f