Question

Assign oxidation number to each element in the following compounds or ions.

i. KMnO₄ 

ii. K₂Cr₂O₇

iii. Ca₃(PO₄)₂

Answer 1

i. KMnO₄

Oxidation number of K = +1 

Oxidation number of O = -2 

KMnO₄ is a neutral molecule. 

∴ Sum of the oxidation numbers of all atoms = 0 

∴ (Oxidation number of K) + (Oxidation number of Mn) + 4 × (Oxidation number of O) = 0

∴ (+1) + Oxidation number of Mn + 4 × (-2) = 0 

∴ Oxidation number of Mn + 1 – 8 = 0 

∴ Oxidation number of Mn – 7 = 0 

∴ Oxidation number of Mn in KMnO₄ = +7

ii. K₂Cr₂O₇

Oxidation number of K = +1

Oxidation number of O = -2

K₂Cr₂O₇ is a neutral molecule.

∴ Sum of the oxidation numbers of all atoms = 0

∴ 2 × (Oxidation number of K) + 2 × (Oxidation number of Cr) + 7 × (Oxidation number of O) = 0

∴ 2 × (+1) + 2 × (Oxidation number of Cr) + 7 × (-2) = 0 

∴ 2 × (Oxidation number of Cr) + 2 – 14 = 0 

∴ 2 × (Oxidation number of Cr) – 12 = 0 

∴ 2 × (Oxidation number of Cr) = +12 

∴ Oxidation number of Cr = +12/2

∴ Oxidation number of Cr in K₂Cr₂O₇ = +6

iii. Ca₃(PO₄)₂

Oxidation number of Ca = +2 

(∵ Ca is alkaline earth metal.) 

Oxidation number of O = -2

Ca₃(PO₄)₂ is a neutral molecule.

Sum of the oxidation numbers of all atoms = 0 

∴ 3 × (Oxidation number of Ca) + 2 × (Oxidation number of P) + 8 × (Oxidation number of O) = 0

∴ 3 × (+2)+ 2 × (Oxidation number of P)+ 8 × (-2) = 0 

∴ 2 × (Oxidation number of P) + 6 – 16 = 0 

∴ 2 × (Oxidation number of P) – 10 = 0 

∴ 2 × (Oxidation number of P) = +10 

∴ Oxidation number of P = +10/2 

∴ Oxidation number of P in Ca₃(PO₄)₂ = +5