Calculate the pZn of a solution prepared by mixing 25.0 mL of 0.0100 M EDTA with 50.0 mL of 0.00500 M Zn2 . Assume that both the Zn2 and EDTA solutions are buffered with 0.100 M NH3 and 0.176 M NH4Cl.

Explanation:

Using the approach of Henderson-HasselBalch equation, we have :

where;

the pKa of = 9.26

concentration of = 0.100 M

concentration of = 0.176 M

∴

the pH of the buffered solution is :

The Chemical equation for the reaction of and EDTA is :

Here;

denotes the fully deprotonated form of the EDTA

The formation constant of the equation for the reaction can be represented as:

     ----- (1)

The logarithm of the formation constant of Zn - EDTA complex = 16.5

 =

 =

Since the formation constant in the above equation signifies that the EDTA is present in  ,

Then:

From (1)

∴

where;

= conditional formation constant

= the fraction of EDTA that exit in the form of the presences of the 4 charges .

So at equivalence point :

all the initially in titrand  is now present in

Obtaining the data for the value of at the reference table:

 =  

∴

To calculate the moles of  EDTA ,  , ; we have:

moles of  EDTA = 0.0100 M × 0.025 L

moles of  EDTA =

moles of = 0.00500 M  × 0.050 L

moles of =

moles of    =  

moles of    =

moles of    =

moles of    = 0.0033333 M

Recall that:

Assume Q² is the amount of complex dissociated in

i.e

Q = M

∴