6. Calculate the molar mass of the following substances.

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


A molar mass is the mass of one mole of a molecule. Here 1 mole of ethyne C2H2 has 2 moles of a carbon atom and 2 moles of the hydrogen atom.

Weight of 1 mole of an atom equals to its atomic weight in gram.

Therefore, the molar mass of C2H2 will be 2*14 + 2*2 = 30

We can calculate the molar mass of other molecules in the same manner. So the answer will be:

a) Ethyne (C2H2) = 2*14 + 2*2 = 30

b) Sulphur molecule (S8) = 8*32 =256

c) Phosphorus molecule(P4) = 4*31 =124

d) Hydrochloric acid (HCl) = (1*1) + (1*35.5) = 36.5

e) Nitric acid (HNO3) = (1*1) + (1*14 )+ (3*16) = 63

Note: In the exam, the atomic mass may be given or may not be given. A simple rule is to remember the atomic number and atomic masses. Most of the time atomic mass is slightly above the double of the atomic number. A student should remember the atomic masses up to the calcium atom(20). Best way to remember is to keep the periodic table handy and revise them at regular interval. Below I have given the atomic number and masses up to 20 elements.

H(1,1); He(2,4); Li(3,7); Be(4,9); B(5,11); C(6,12); N(7,14); O(8,16); F(9,19); Ne(10,20), Na(11,23), Mg(12,24); Al(13,27), Si(14,28); P(15,31); S(16,32); Cl(17,35.5) Ar(18,40), K(19,40), Ca(20, 40).

Next: 7. What is the mass of—(a) 1 mole of nitrogen atoms? (b) 4 moles of aluminium atoms (Atomic mass of aluminium= 27)? (c) 10 moles of sodium sulphite (Na2SO3)?

See also: Question 4 and 5: Write the chemical formulae of the following; Give the names of the elements present in the following compounds.

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