Question

The atomic mass of deuterium isotope of hydrogen is 2. The atomic mass of oxygen is 16. Then what is the molecular mass of heavy water?
(A) 16
(B) 18
(C) 20
(D) 24

Answer 1

Hint: Two or more atoms have the same atomic number and different mass numbers then they are called isotopes. Hydrogen has three isotopes, hydrogen, deuterium and tritium. They can be represented as follows.
Hydrogen - \[{}_{1}^{1}H\], Deuterium- \[{}_{2}^{1}H\], and Tritium-\[{}_{3}^{1}H\]. They have the same atomic number but different mass numbers.

Complete step by step solution:
-In the question it is given that the atomic mass of deuterium is 2, and the atomic mass of oxygen is 16.
-The molecular formula of heavy water is\[{{D}_{2}}O\].
-Heavy water contains two deuterium atoms and one oxygen atom.
-The deuterium has one neutron and one proton in its nucleus.
-Molecular mass of heavy water (\[{{D}_{2}}O\]) \[\begin{align}
& \text{=(2 }\!\!\times\!\!\text{ Atomic mass of Deuterium + Atomic mass of Oxygen)} \\
& \text{= (2}\times \text{2)+16} \\
& \text{=20} \\
\end{align}\]
-Therefore the molecular mass of heavy water = 20.

So, the correct option is C.

Additional information:
-Heavy water is used in nuclear reactors and it is used in Nuclear Magnetic Resonance (NMR) spectroscopy.
-Heavy water is used in the preparation of another isotope of hydrogen called tritium.
-Heavy water is also called deuterium oxide.
-Though Deuterium is an isotope of hydrogen, Heavy water is not useful for drinking purposes.

Note: Atomic mass is different from atomic number. Atomic number represents the number of electrons present in the atom and atomic represents the number of protons and the number of neutrons present in the nucleus of the atom.