In: Chemistry
Solutions of chromate are yellow in colour. At concentrations above 2.5 × 10-3 M, the sample solution is sufficiently coloured such that the observance of the red precipitate is masked. Calculate the relative error in the volume of titrant added that would be observed for the titration of an unknown chloride solution containing 2.5 × 10-3 M chromate with 0.1 M AgNO3. Assume that 25 mL of titrant is required to reach the endpoint, where the total solution volume will be 1 litre.
We have,
Ksp of AgCl = 1.82 x 10-10
Therefore,
Concentration of Ag at the equivalence point = (1.82 x 10-10)1/2 = 1.34 x 10-5M
Also,
Ksp of Ag2CrO4 = 1.1 x 10-12
Therefore,
[Ag+]2 = Ksp/[CrO42-] = (1.1 x 10-12)/(2.5 x 10-3) = 4.4 x 10-10
[Ag+] = 2.1 x 10-5 M
Error in concentration = Moles of excess Ag = 2.1 x 10-5 – 1.34 x 10-5 M = 7.6 x 10-6 M
Error in the volume of titrant added = [(Moles of excess Ag)/Concentration of titrant] x 1000ml/L
= [(7.6 x 10-6 M)/0.1 M] x 1000 mL/L = 0.08 mL
If 25 mL was used to reach the endpoint, then
Percent error = (0.08/25) x 100% = 0.32 %