Question

Two oxides of a metal (M) have 20.12% and 11.19% oxygen. The formula of the first oxide is MO. Determine the formula of the second oxide.

Answer 1

Calculation of molar mass of M from first oxide :
Let us assume the atomic mass of Mas x.
Atomic mass of Oxygen = 16

Element	Percentage	Relative number of  moles	Simplest mole ratio
M	100 - 20.12 = 79.88	79.88/x	79.88/1.25x
O	20.12	20.12/16=1.25	1.25/1.25 = 1

So, molar mass of M, x = 63.5
Calculation Of formula of second oxide :

Element	Atomic mass	Percentage	Relative number of moles	Simplest  mole ratio	Whole number ratio
M	63.5	88.81	88.81/63.5 = 1.4	1.4/0.69 = 2	2
O	16	11.19	11.19/16 = 0.69	0.69/0.69 =1	1

So formula of second oxide is M₂O.