How to calculate molecular mass

Molecular mass is the sum of atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together. For e.g. molecular mass of H₂SO₄ is calculated as: 2 x 1 + 32 + 4 x 16 = 98 u.

In the case of ionic compounds (like NaCl), there is no discrete (separate) molecules. Here the positive ions and the negative ions are arranged in a three-dimensional structure. So we can calculate only formula mass by taking molecular formula of the compound.

Calculating Molecular Mass:-

The molecular mass of a substance is calculated by adding up the atomic masses of all the atoms in the molecule. The atomic mass of an element is the mass of a single atom of that element, expressed in amu.

For example, the molecular mass of water (H₂O) can be calculated as follows:

Molecular mass of water (H₂O) = (2 x atomic mass of hydrogen) + (1 x atomic mass of oxygen) = (2 x 1.008 amu) + (1 x 16 amu) = 18 amu

This means that one molecule of water has a molecular mass of 18 amu.

Molecular mass is not the same as molar mass: Although the terms “molecular mass” and “molar mass” are often used interchangeably, they actually refer to slightly different concepts. Molecular mass is the mass of a single molecule, while molar mass is the mass of one mole of a substance. However, since one mole of a substance contains Avogadro’s number of molecules, the two concepts are related: the molar mass of a substance is equal to its molecular mass multiplied by Avogadro’s number.

Molecular mass of Simple Covalent Compound

1.Water (H₂O):- 2 + 16 = 18 u

2. Ammonia (NH₃) :- 14 + 3 = 17 u

3. Methane (CH₄) :- 12 + 4 = 16 u

4. Carbon Dioxide ( CO₂) :- 12 + 32 = 44 u

5. Carbon Monoxide ( CO ) :- 12 + 16 = 28 u

6. Ozone (O₃) :- 48u

7. Dinitrogen ( N₂) :- 14 + 14 = 28u

8. Dioxygen ( O₂) :- 16 + 16 = 32

9. Hydrochloric acid (HCl) :- 1 + 35.5 = 36.5 u

10. Nitrogen Dioxide ( NO₂) :- 14 + 32 = 46 u

11. Nitrogen Monoxide (NO) :- 14 + 16 = 30 u

12. Phosphorus molecule (P₄) :- 4 x 31 = 124 u

13. Sulphur molecule (S₈) :- 8 x 32 = 256 u

14. Sulphur Dioxide (SO₂) :- 64 u

15. Sulphur Trioxide (SO₃) :- 80 u

Molecular Mass Quiz -1

16. Ethane ( C₂H₆) :- 24 + 6 = 30 u

17. Butane ( C₄ H₁₀) :- 48 + 10 = 58 u

18. Dihydrogen(H₂):- 1+1 = 2 u

19 Chlorine gas (Cl₂) :- 2 x 35.5 = 71 u

20. Argon gas (Ar):- 40

• -Molecular mass (or molecular weight) is the mass of a single molecule of a substance. It is calculated by adding up the atomic masses of all the atoms in the molecule.

• -Molar mass, on the other hand, is the mass of one mole of a substance. It is expressed in units of grams per mole (g/mol) and is numerically equal to the molecular mass in atomic mass units.

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