Question

(a) Define mole. What are the two things that a mole represents.
(b) What weight of each element is present in 1.5 moles of sodium sulphite, Na₂SO₃?

(Atomic masses : Na = 23 u ; S = 32 u ; O = 16 u)

Answer 1

(a) Mole can be defined as, the amount of a pure substance, containing the same number of atoms, molecules or ions as present in 12g of carbon. This refers to the value of 6.022 × 10²³ atoms, molecules or ions of a substance.
A mole represents Avogadro number of particles of a substance.

(b)
1.5 moles of sodium sulphite
Molar mass of sodium sulphite = (2 × 23) + (1 × 32) + (3 × 16) = 126g
Therefore, 1 mole of sodium sulphite weighs 126g.
Now, 1 mole of sodium sulphite contains 2 moles of sodium.
So, mass of sodium in 1 mole of sodium sulphite = 2 × 23 = 46g
Therefore, mass of sodium in 1.5 moles of sodium sulphite is = 1.5 × 46 = 69g

1 mole of sodium sulphite contains 1 mole of sulphur.
So, mass of sulphur in 1 mole of sodium sulphite = 1 × 32 = 32g
Therefore, mass of sulphur in 1.5 moles of sodium sulphite is = 1.5 × 32 = 48g

1 mole of sodium sulphite contains 3 moles of oxygen.
So, mass of oxygen in 1 mole of sodium sulphite = 3 × 16 = 48g
Therefore, mass of oxygen in 1.5 moles of sodium sulphite is = 1.5 × 48 = 72g
Hence, the mass of sodium, sulphur and oxygen in sodium sulphite is 69g, 48g and 72g, respectively.