Calculate the formula masses of the following compounds :
(i) Sodium oxide, Na₂O
(ii) Aluminium oxide, Al₂O₃
(Given : Atomic masses : Na = 23 u ; O = 16 u ; Al = 27 u)

(i) Formula mass of Na₂O:

Formula mass of Na₂O = Mass of two Na atoms + Mass of one O atom

= 2 × 23 + 1 × 16
= 46 + 16
= 62

Therefore, the formula mass of Na₂O is 62 u.

(ii) Atomic mass of aluminium = 27 u

Atomic mass of oxygen = 16 u
Formula mass of Al₂O₃ can be calculated as follows:
Formula mass of Al₂O₃ = Mass of two Al atoms + Mass of three O atoms

= 2 × 27 + 3 × 16
= 54 + 48
= 102

Therefore, the formula mass of Al₂O₃ is 102 u.