Question 1

Calculate the molecular masses of H₂, O₂, Cl₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃OH.

Answer

Atomic mass includes the masses of three subatomic particles which make up an atom i.e. proton, neutron and electron.

Unit of atomic mass is 'u'

‘u’ = unified atomic mass unit & 1/12 th mass of single carbon-12 atom

‘u’ = 1.66 x 10^-24 g

(Just for understanding - Atomic mass approximately equal to twice of atomic no.)

Molecular mass of a substance is the sum of atomic masses of all the atoms in a molecule of a subitance.

Unit of molecular mass = 'u’

Molecular mass of H₂ = 2 x 1 = 2u (Atomic mass of H=1)

Molecular mass of O₂ = 16 x 2 = 32 u (Atomic mass of O=16)

Molecular Mass of Cl₂ = 35.2 x 2 = 71 u (Atomic mass of Cl = 35.5)

Molecular Mass of CO₂ = 12 x 1 + 16 x 2

= 12 + 32 = 44 u ( Atomic Mass of C = 12)

Molecular Mass of CH₄ = 12 x 1 + 4 x 1

= 12 + 4 = 16 u

Molecular Mass of C₂H₆ = 12 x 2 + 6 x 1

= 24 + 6 = 30 u

Molecular Mass of C₂H₄ = 12 x 2 + 4 x 1

= 24 + 4 = 28 u

Molecular Mass of NH₃ = 14 X 1 + 3 x 1

= 14 x 3 = 17 u (Atomic Mass of N = 14)

Molecular Mass of CH₃OH = 12 x 1 + 4 x 1 + 16 x 1

= 12 + 4 + 16 = 32 u

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