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 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).
