Heavy water, $D_2O$ have molecular mass of _____.

20

The correct answer is option 1. 20.

The molecular mass of heavy water, \(D_2O\), can be calculated by summing the atomic masses of its constituents.

The atomic mass of deuterium (\(D\)) is approximately \(2 \, \text{amu}\) (as it is a heavier isotope of hydrogen).

The atomic mass of oxygen (\(O\)) is approximately \(16 \, \text{amu}\).

Therefore, the molecular mass of heavy water (\(D_2O\)) is:

\[2 \times (\text{Atomic mass of deuterium}) + \text{Atomic mass of oxygen} = 2 \times 2 \, \text{amu} + 16 \, \text{amu} = 4 \, \text{amu} + 16 \, \text{amu} = 20 \, \text{amu}\]

So, the correct answer is 1. \(20\).