Question

Find pZn²⁺ after adding 30.0 and 51.0 mL of EDTA.

Short answer

After adding 30 mL EDTA pZn+ is 8.35 and after adding 51 mL of EDTA pZn+ is 10.3

Step-by-step solution

Information given

Consider the titration of 50.0 mL of 1.00 × 10^-3 M Zn²⁺ with 1.00 × 10^-3 M EDTA at

pH 10.00 in the presence of 0.10 M NH₃. The equivalence point is at 50.0 mL

Equations and data obtained in order to proceed for calculation are as follows

${\alpha }_{Z{n}^{2+}}=\frac{1}{1+{\beta }_{1}L+{\beta }_2{L}^2+{\beta }_3{L}^3+{\beta }_4{L}^4}$

From Eq 12-17${\alpha }_{Z{n}^{2+}}=1.8\times {10}^{-5}$

$$\begin{gathered} K_f={10}^{16.5} \\ \mathrm{At}\mathrm{pH}10{\alpha }_{Y^{4-}}=0.3(Table12-1) \end{gathered}$$

Determine equilibrium constant

$\begin{array}{l}{K}_{{}_4}^{\text{'}}={\alpha }_{Z{n}^{2+}}\times {\alpha }_{y^{4-}}\times K_f\\ =1.8\times {10}^{-5}\times 0.3\times {10}^{16.5}\\ =1.7\times {10}^{11}\end{array}$

Equivalence point=50 mL

Determine the value of pZn2+ after adding 30 mL of EDTA

Before the equivalence point

The concentration of the remaining productcan be calculated using the following equation

=Fraction remaining × Initial concentration × Dilution factor

If 30 mL solution is added then the reaction will be 30/50 completed as the equivalence point is at 50 mL. Then the metal concentration will be