Question

10.00 ml of seawater containing Mg2+ ions is determined by precipitation with an oxalate (C2O42-) solution. Mg2+ + C2O42- <> MgC2O4 The precipitate was filtered, redissolved, and titrated to endpoint requiring 18.21 ml of a standard MnO4-. Beforehand, the MnO4- solution was standardized with 10.00 ml of a 2.00 * (10)-3 M oxalate solution and required 32.10 ml of MnO4- solution. Calculate the concentration of Mg2+ in the seawater sample. 2 MnO4- + 5 C2O42- + 6H+ ==> 10 CO2 + 2 Mn2+ + 8 H2O

Answer 1

Since it a a redox titration, it is better to use normality for all the concentrations than the molarity.

For converting molarity to normality conversion, we use the following formula.

normality (N) = n-factor x molarity; where n-factor = no. of electrons gained or lost by a species in redox reaction.

Standardizing MnO₄^- ​solution:

given redox reaction 2MnO₄^- + 5 C₂O₄^2- + 6H+ ​ 2 Mn2+ + 10 CO2 + 8H2O

MnO4- getting reduced for =7 oxidation state to +2 state. i.e. it is gaining 5 e^-.

Hence n-factor for MnO₄^- = 5

given volume of MnO₄^- (V₁) = 32.10 ml

Similarly C₂O₄^2- is getting oxidised from +6 to +8 oxidation state. i.e. loosing 2e^-

hence n-factor of C₂O₄^2- = 2

givenmolarity of C₂O₄^2- (M₂) = 2 x 10^-3 M

Normality of C₂O₄^2- (N₂) = 2x2x 10^-3 N = 4 x 10^-3 N

volume of C₂O₄^2- (V₂) = 10 ml

from the law of equivalents

no. of milli equivalents of MnO₄^- = no. of milli equivalents of C₂O₄^2-

i.e. V₁N₁ = V₂N₂​N₁ = V₂N₂/ V₁

​ N₁ = 10 x 4 x 10^-3 / 32.10 = 1.2461 x 10 ^-3 N

Estimating concentration of Mg²⁺:

​given Mg²⁺ + C₂O₄^2-​ MgC₂O₄

from the reaction no. of moles of Mg²⁺ = no. of moles of C₂O₄^2-

i.e. normality of Mg2+ in the solution = normality of C₂O₄^2-

normality of C₂O₄^2- can be calted as follows.

volume of MnO4- (V₁) = 18.21 ml

normality of MnO4- (N₁) = 1.2461 x 10^-3 N

Volume of C₂O₄^2- (V₂) = 10.0 ml

normality of C₂O₄^2- = N₂

N₁V₁ = N₂V₂​ N₂ = N₁V₁ / V₂

hence N₂ = 1.2461 x 10^-3 X 18.21 / 10 = 2.2691 x 10^-3 N

Hence concentration of Mg²⁺ in sea water = 2.2691 X 10^-3 N