Question

Determine the Hydrolysis of poly valent anions or cations.

Answer 1

The hydrolysis of these species will take place in steps (just like dissociation of weak acids). 

Out of different steps generally first step hydrolysis dominants mainly because of two reasons.

The hydrolysis constant of second and further steps is generally negligible in comparison to first step hydrolysis constant.

The second and further step hydrolysis will be suppressed in presence of ions produced due to first step hydrolysis. 

For a polyprotic acid (H₂S, H₃PO₄, H₂CO₃, H₂C₂O₄) we already know that the dissociation always takes place in steps, so for example for H₃PO₄ 

For all acids we always have Ka₁ >> Ka₂ >> Ka₃

pH of the solution can be calculated from Ist step only because [H+ ] from II^nd & III^rd step can be neglected as 

(a) Ka₁ >> Ka₂ >> Ka₃ 

(b) [H⁺ ] from I^st dissociation will suppress the dissociation of II^nd & III^rd step. Now for the hydrolysis of polyvalent ions of salts (like K₃PO₄, Na₂C₂O₄, ZnSO₄, FeCl₃, (NH₄)₂C₂O₄ or ions like PO₄^3–, C₂O₄^2–, Zn²⁺, Fe³⁺ etc).

Consider the hydrolysis in step