Question

A 110.0 mL sample of 0.040 M Ca2+ is titrated with 0.040 M EDTA at pH 9.00. The value of logKf for the Ca2+−EDTA complex is 10.65 and the fraction of free EDTA in the Y4− form, ????Y4− , is 0.041 at pH 9.00. What is K′f , the conditional formation constant, for Ca2+ at pH 9.00?

Answer 1

Answer : The conditional formation constant is $1.83\times 10^9$

Explanation :

First we have to calculate the formation constant.

As we are given that:

$\log k_f=10.65$

$k_f=10^(10.65)$

$k_f=4.47\times 10^{10}$

Thus, the formation constant is $4.47\times 10^{10}$

Now we have to calculate the conditional formation constant.

The expression used as:

$k'_f=\alpha_y^+\times k_f$

where,

$k'_f$ = conditional formation constant = ?

$\alpha_y^+$ = activity coefficient at pH 9.00 = 0.041

$k_f$ = formation constant = $4.47\times 10^{10}$

Now put all the given values in the above expression, we get:

$k'_f=(0.041)\times (4.47\times 10^{10})$

$k'_f=1.83\times 10^9$

Therefore, the conditional formation constant is $1.83\times 10^9$