Question

A titration of Cu2+ with EDTA (abbreviated Y4-) is performed using an Erio-Black T indicator. The titration has an error of +0.10% (if the titration should have used 100 mL to reach the equivalence point, it actually used 100.1 mL because of the end point error. Assume the titration involved 100 mL of 0.01 M Cu2+ and a 0.02 M EDTA titrant. The reaction is

Cu2+ + Y4- → CuY2- K = 2 x 109

a) What volume of EDTA was actually used to reach the equivalence point?

b) What volume of EDTA was actually used to reach the end point?

c) What are the analytical concentrations of Y4- and CuY2- at the end point?

d) What is the equilibrium concentration of Cu2+ at the end point?

e) What are the equilibrium concentrations of Y4- and CuY2- at the end point?

f) What should the concentration of Cu2+ be at the true equivalence point?

Answer 1

Cu2+ + Y4- < --------> CuY2-

a) The reaction is 1:1 molar reaction i.e 1 mole of Cu2+ react with 1mole of EDTA, so

M1× V1 = M2 × V2

0.01 × 100ml = 0.02 × V2

V2 = 0.01 × 100ml / 0.02

= 50ml

b) if consider the 10% error in end point

Volume of EDTA consumed = 50.05ml

c) Analytical concentration of Y4- = 0

Analytical concentration of CuY2- = (0.001mol/150.05ml)×1000ml = 0.00666M

d) CuY2- <-------> Y4- + Cu2+ K=5×10^-10

5×10^-10 = X^2/ (0.00666)

X = 1.824×10^-6M

Therefore

[ Cu2+ ] at equillibrium = 1.824×10^-6M

e) [ CuY2- ] = 0.00666M

[ Cu2+ ] = 1.824×10^-6M

[ Y4-] = 1.824 ×10^-6M