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Class 9 Science - Chapter Atoms and Molecules NCERT Solutions | Calculate the molecular masses of H2, O2

Welcome to the NCERT Solutions for Class 9th Science - Chapter Atoms and Molecules. This page offers a step-by-step solution to the specific question from Excercise 4 , Question 1: calculate the molecular masses of h2 o2 cl2 co2....
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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