(i) Magnesium bicarbonate:

Valency of magnesium = 2

Valency of carbonate = 2

By applying criss-cross method:

Formula: MgCO₃

(ii) Cupric oxide:

Valency of cupric = 2

Valency of oxide = 2

By applying criss-cross method:

Formula: CuO

(iii) Ferric oxide:

Valency of ferric = 3

Valency of oxide = 2

By applying criss-cross method:

Formula: Fe₂O₃

(iv) Ammonium hydroxide

Valency of ammonium = 1

Valency of hydroxide = 1

By applying criss-cross method:

Formula: NH₄OH

(v) Calcium carbonate

Valency of calcium = 2

Valency of carbonate = 2

By applying criss-cross method:

Formula: CaCO₃

(vi) Potassium carbonate

Valency of potassium = 1

Valency of carbonate = 2

By applying criss-cross method:

Formula: K₂CO₃