Chapter 3: Atoms and Molecules

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Atoms and Molecules

Question:

Q.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:

(a) The ethyne (C2H) contains carbon and hydrogen atoms. The gram atomic mass of carbon is 12 g and hydrogen is 1 g.

So, Molar mass of C2H2 = 2(Atomic mass of C) +2(Atomic mass of H)

= 2(12g) + 2(1g) = 26g

Therefore, molar mass of ethyne is 26g.

 

(b) The octasulphur molecule(S8 ) contains eight sulphur atoms. The gram atomic mass of sulphur is 32 g.

So, Molar mass of S8 = 8(Atomic mass of S)

= 8 (32g) = 256g

Therefore, molar mass of octasulphur molecule is 256g.

 

(c) The tetraphosphorus (P) contains four phosphorus atoms. The gram atomic mass of phosphorus is 31 g.

Molar mass of P4 = 4(Atomic mass of P)

= 4 (31g) = 124g

Therefore, molar mass of tetraphosphorus molecule is 124g.

 

(d) The hydrogen chloride (HCl ) contains chlorine and hydrogen atoms. The gram atomic mass of chlorine is 35.5 g and hydrogen is 1 g.

So, Molar mass of HCl = Atomic mass of H +Atomic mass of Cl

= 1g + 35.5g = 36.5g

Therefore, the molar mass of hydrogen chloride is 36.5g.

 

(e) The nitric acid(HNO) contains hydrogen, nitrogen, and oxygen atoms. The gram atomic mass of nitrogen is 14 g, oxygen is 16 g and hydrogen is 1 g.

So, Molar mass of HNO3 = Atomic mass of H + Atomic mass of N + 3(Atomic mass of O)

= 1g + 14g + 3(16g) = 63g

Therefore, the molar mass of nitric acid is 63g.


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