Question

What is the mass of 1 molecule of ${O_2}$ in a.m.u. ?

Answer 1

Hint: Molar mass of oxygen is 32. In one mole of oxygen there are one Avogadro number of oxygen molecules ($6.023 \times {10^{23}}$). A.m.u. stands for atomic mass unit.

Complete step-by-step answer:
Step 1:
An atom is the smallest particle of an element that can take part in a chemical reaction. An atom may or may not exist independently.
A molecule is the smallest particle of an element or a compound that has an individual existence.
Atomicity is the number of atoms present in a molecule.

Step 2:
The atomic mass of an element which is expressed in grams is known as gram atomic mass.
The molecular mass of a substance expressed in grams is known as gram molecular mass or molar mass.

Step 3:
As it is not practically possible to find the mass of a very small particle like atoms and molecules, so a collection of $6.023 \times {10^{23}}$ elementary particles (Atoms or molecules) known as mole is taken for this purpose.
Avogadro’s number is the number is the number of atoms, ions or molecules present in one mole of the substance.

Step 4:
The gram molecular mass of oxygen is 32gms. This means that the mass of one mole of oxygen is $32g$.
So, the mass of $6.023 \times {10^{23}}$molecules of oxygen is $32g$.
Therefore, the mass of one molecule of oxygen $= \dfrac{{32}}{{6.023 \times {{10}^{23}}}}g$
Mass of one molecule of oxygen $= 5.31 \times {10^{ - 23}}g$

Step 5:
In an atomic mass unit, the mass of one atom of oxygen is $16a.m.u.$ Oxygen is a diatomic gas. So, in a molecule of oxygen there are two atoms. Therefore, the molecular mass of oxygen in atomic mass unit is $= 2 \times 16 = 32a.m.u.$

Hence the mass of one molecule of oxygen is = 2 \times 16 = 32a.m.u.$$$$ = 5.31 \times {10^{ - 23}}g

Note: Molecular mass is the mass of Avagadro’s number of molecules whereas Mass of a molecule is that of a single molecule also known as its actual mass.