How to determining the oxidation state of an element in a compound?

Determining oxidation state of an atom in a compound

Example : A device called Breathalyzer is used by police to test a person’s health for alcohol. It contains an acidic solution of potassium dichromate K2Cr2O7. It is a strong oxidizing agent. Determine oxidation state.

Important Note

Remember, before determining oxidation state, one must know oxidation state of elements in following table.

Solution

Oxidation state of K = +1

Oxidation state of O = -2

Oxidation state of Cr = X

Since, the algebraic sum of oxidation state of all atoms in a neutral molecule or formula unit is zero. So,

2(Oxidation state of K) +2(Oxidation state of Cr) + 7(Oxidation state of O) = 0

2(+1) +2 (Oxidation state of Cr)+ 7(-2) = 0

+2 +2 (X) -14 =0

2 (X) -12 =0

2 (X) =+12

X = +6

Oxidation state of Cr = +6

Example: Boric acid (H3BO3) is used in eye wash . What is the oxidation state of B in this acid?

Solution

Oxidation state of H= +1

Oxidation state of O = -2

Oxidation state of B = X

Since, the algebraic sum of oxidation states of all atoms in a neutral molecule or formula unit is zero.

3(Oxidation state of H) + (Oxidation state of Cr) +3(Oxidation state of O) = 0

3(+1) +(Oxidation state of B)+ 3(-2) = 0

+3 + (X) -6 =0

X -3 =0

X =+3

Oxidation state of B =+3

Problem, Determine the oxidation state of nitrogen in the following compounds.

i. NO2 iv. N2O iv. N2O3 iv. HNO3

i. NO2

Solution

Oxidation state of O = -2

Oxidation state of N = X

Since, the algebraic sum of oxidation states of all atoms in a neutral molecule or formula unit is zero. So,

(Oxidation state of N) + 2(Oxidation state of O) = 0

(X)+ 3(-2) = 0

X -6 =0

X =+6

Oxidation state of N =+6

ii. N2O

Solution

Oxidation state of O = -2

Oxidation state of N = X

Since, the algebraic sum of oxidation states of all atoms in a neutral molecule or formula unit is zero. So,

2(Oxidation state of N) + (Oxidation state of O) = 0

2(X)+ (-2) = 0

2X -2 =0

X =+2/2 = +1

Oxidation state of N =+1

iii. N2O5

Solution

Oxidation state of O = -2

Oxidation state of N = X

Since, the algebraic sum of oxidation states of all atoms in a neutral molecule or formula unit is zero. So,

2(Oxidation state of N) + 3(Oxidation state of O) = 0

2(X)+3 (-2) = 0

2X -6 =0

X =+6/2 = +3

Oxidation state of N =+3

iii. HNO3

Solution

Oxidation state of H = +1

Oxidation state of O = -2

Oxidation state of N = X

Since, the algebraic sum of oxidation states of all atoms in a neutral molecule or formula unit is zero. So,

(Oxidation state of H) + (Oxidation state of N) + 3(Oxidation state of O) = 0

(+1) + (X)+3 (-2) = 0

+1+ X -6 =0

X -5 =0

X = +5

Oxidation state of N =+5

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