Redox Titrations (Unknown Oxidation No.)

 

 

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  1. Determine the reacting ratio between the reactants.
  2. Construct the following table:

     

     

    Rxt A

    Rxt B

    Reacting ratio

    P

    R

    n of e– transferred per mol of reactant

    (= change in ON)

    Q

    S

    Total n of e– transferred (P x Q or R x S)

    P x Q = R x S

     

  3. Since total n of e- transferred must be the same for both reactants, make use of the relation (PxQ = RxS) to obtain the number of electrons transferred by the reactant with the unknown ON (Q or S).
  4. Consider whether the reactant had undergone oxidation or reduction; Add or subtract Q or S accordingly to get initial/ final unknown oxidation state.

 

Example:

100 cm3 of 0.20 mol dm–3 acidified Fe2+ is oxidized by 50 cm3 of a solution containing 0.20 mol dm–3 of MO3–. Determine the final oxidation state of M.

 

Fe2+

MO3–

Reacting ratio

2

1

n of e– transferred per mol of reactant

1

Fe2+ → Fe3+ + 1e– 

?

Total n of e– transferred

2

  • Using the relation P x Q = R x S, n of e– transferred per mol of MO3– = 2
  • Fe2+ is oxidised so MO3– must be reduced.
  • Initial ON of M in MO3– is +5 → Final ON = +5 – 2 = +3

 

Practice:

0.5 g of Zn was reacted with an acidified solution of 25.50 cm3 of 0.200 mol dm–3 MO2+. What is the final oxidation number of metal M?

[Ans: –1]