Question

1. A 100.00 mL solution of 0.0500 M Cu2+ was buffered to pH 9.00 and titrated with 0.0650 M EDTA

( a) What volume of EDTA is needed to reach the equivalence point?

(b) If the conditional formation constant for the Cu-EDTA complex at pH = 9.00 is K’f = 2.47 x 1017, calculate the [Cu2+] at the equivalence point.  

Answer 1

a) In given question following reaction takes place :

Cu²⁺+ (EDTA)^2- [Cu(EDTA)]^2- + 2H⁺

This H⁺ released from EDTA

this equation shows that 1equivalent of Cu²⁺ reacts with 1equivalent of EDTA. So at equivalence point,

equivalents of Cu²⁺ = equivalents of EDTA

or. N₁V_{1( for Cu²⁺)} = N₂ V_{2 ( for EDTA)}

Here N₁ =2* 0.05 =0.1N ( N= n-factor * Molarity) V₁=100ml

N₂=2*0.065=0.13N. V₂=?

Substituting these values we get , V₂ = 0.1*100/0.13 = 77ml

b) K_f, conditional formation constant=

[Cu(EDTA)^2-]/{[Cu²⁺]*[EDTA]}

Now at equivalence point all Cu²⁺ is in the form of Cu(EDTA)^2- , its concentration is given by

= moles of Cu²⁺ added/ total volume of solution

=0.05*100/ 100+77

=5/177 moles/ml

=28.2M

Let [Cu²⁺] =[EDTA] =x

So, K_f =28.2/x² =2.47 *10¹⁷

x=(2.47*10¹⁷/28.2)^1/2

=9.3* 10⁷M