10.0 mL of a Cu2+ solution of unknown concentration was placed in a

10.0 mL of a Cu2+ solution of unknown concentration was placed in a 250 mL Erlenmeyer flask. An excess of KI solution was added. Indicator was added and the solution was diluted with H2O to a total volume of 75 mL. The solution was titrated with 0.20 M Na2S2O3. The equivalence point of the titration was reached when 16.20 mL of Na2S2O3 had been added. What is the molar concentration of Cu2+ in the unknown solution?

3 answers

2Cu^2+ + 4I^- ==> 2CuI + I2
Then I2 + 2S2O3^2- ==> S4O6^2- + 2I^-

mols S2O3^2- = M x L = ?
Mols I2 = 1/2 that from the equation that 2 mol S2O3^2- = 1 mol I2.
mols Cu = 2x mols I2 from the first equation = ?

Then M Cu^2+ solution is mols Cu/volume = mols Cu/0.010 L

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