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Question 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

Answer

Molar mass: The mass of the mole of substance is known as molar mass.

Unit = gm/mol.

(a) C2H2 = H - C = C - H

     = 2 x Atomic mass of C + 2 x Atomic mass of H
     = 2 x 12 + 2 x 1
     = 24 + 2 = 26 g


(b) S8 =  8 x atomic mass of sulphur.
      = 8 x 32 = 256 g
 

(c) P4 = 4 x atomic mass of phosphorus

     = 4 x 31 = 124 g

(d) HCl = 1 x atomic mass of H + 1 x atomic mass of Cl

      = 1 x 1 + 1 x 35.5
      = 1 + 35.5 = 36.5 g

(e) HNO3 = 1 x atomic mass of H + 1 x atomic mass of N + 3 x atomic mass of O

      = 1 x 1 + 1 x 14 + 3 x 16
      = 1 + 14 + 48 = 63 g

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