Question

Write the chemical formula and formula unit masses of:
(i) magnesium carbonate
(ii) aluminium oxide
[5 Marks]

Answer 1

(i) Magnesium carbonate consists of magnesium ion and carbonate ion, i.e., ${Mg}^{2+}$and ${CO}_3^{2-}$. Both the ions have valency 2.

Symbol	Mg	${CO}_3$
Valency	2	2

Upon criss-crossing the valencies and balancing the charges, we get the formula as $Mg{CO}_3$.


[1.5 Marks]

The formula unit mass of $Mg{CO}_3$ =
(1 $\times$ atomic mass of Mg) + (1 $\times$ atomic mass of C) + (3 $\times$ atomic mass of O) = (1 $\times$ 24 u) + (1 $\times$ 12 u) + (3 $\times$ 16 u) = 84 u

[1 Mark]

(ii) Aluminium oxide consists of aluminium ion and oxide ion, i.e., ${Al}^{3+}$and $O^{2-}$, having valency 3 and 2 respectively.

Symbol	Al	O
Valency	3	2

Upon criss-crossing the valencies, we get the formula as ${Al}_2{O}_3$.


[1.5 Marks]

The formula unit mass of ${Al}_2{O}_3$ =
(2 $\times$ atomic mass of Al) + (3 $\times$ atomic mass of O) = (2 $\times$ 27 u) + (3 $\times$ 16 u) = 102 u

[1 Mark]