NCERT Solutions for Class 9 Science Chapter 3
Atoms And Molecules
NCERT Books for Session 2022-2023
CBSE Board and UP Board
Exercises Questions
Page No-44
Questions No- 6
Calculate the molar mass of the following substances.
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO3
Molar Mass Calculation:
(a) Ethyne, C2H2:
– Molar mass = 2 x Atomic mass of C + 2 x Atomic mass of H
– Molar mass = 2 x 12g/mol + 2 x 1g/mol = 26g/mol
(b) Sulphur molecule, S8:
– Molar mass = 8 x Atomic mass of S
– Molar mass = 8 x 32g/mol = 256g/mol
(c) Phosphorus molecule, P4:
– Molar mass = 4 x Atomic mass of P
– Molar mass = 4 x 31g/mol = 124g/mol
(d) Hydrochloric acid, HCl:
– Molar mass = Atomic mass of H + Atomic mass of Cl
– Molar mass = 1g/mol + 35.5g/mol = 36.5g/mol
(e) Nitric acid, HNO3:
– Molar mass = Atomic mass of H + Atomic mass of N + 3 x Atomic mass of O
– Molar mass = 1g/mol + 14g/mol + 3 x 16g/mol = 63g/mol
Summary of Molar Masses:
(a) Ethyne, C2H2: 26 g/mol
(b) Sulphur molecule, S8: 256 g/mol
(c) Phosphorus molecule, P4: 124 g/mol
(d) Hydrochloric acid, HCl: 36.5 g/mol
(e) Nitric acid, HNO3: 63 g/mol
These molar masses represent the mass of one mole of each substance, aiding in calculations involving quantities of substances in chemical reactions.
(a) Molar mass of ethyne, C2H2 = 2 × 12 + 2 × 1 = 28g
(b) Molar mass of sulphur molecule, S8 = 8 × 32 = 256g
(c) Molar mass of phosphorus molecule, P4 = 4 × 31 = 124g
(d) Molar mass of hydrochloric acid, HCl = 1 + 35.5 = 36.5g
(e) Molar mass of nitric acid, HNO3 = 1 + 14 + 3 × 16 = 63g
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