Calculation of Hydrolysis of Weak Acid and Strong Alkali Salt

1.
Weak acid and strong base salt

NaCN is a weak acid and strong base salt, completely dissociated in water

CN ^- combines with H ⁺ generated by dissociation of H ₂ O to form a weak electrolyte HCN

The combination of H ⁺ and CN ^- generates a weak electrolyte HCN, which makes the OH ^- concentration in the system greater than the H ⁺ concentration, and the solution appears alkaline
.

The equilibrium constant of the hydrolysis reaction is

If the initial concentration of NaCN is c0, when the hydrolysis reaches equilibrium, there is

Kh ^{Θ is} generally very small
.

Similar to the degree of dissociation, the degree of hydrolysis refers to the fraction of the hydrolyzed CN ^- accounting for the initial concentration of NaCN, usually expressed by h

Salts of polybasic weak acids, such as Na ₂ S, Na ₂ CO ₃ , Na ₃ PO _4, etc.

Since Kh1 ^Θ > Kh2 ^Θ , the OH- concentration in the system is mainly determined by the first step hydrolysis equilibrium
.

[Example 7-7] Calculation mol · DM 0.

The hydrolysis of Na ₂ S in solution is carried out in two steps

Since KH1 ^[Theta] "Kh2 ^[Theta] , so that the solution [OH ^- ] can be calculated first hydrolysis
.

Since Kh1 ^{Θ is} relatively large, it cannot be approximated
.

x=5.

[OH ^- ]=5.

Since Kh2 ^[Theta] is very small, so that the HS ^- the minimum degree of hydrolysis of the second step face can be neglected, i.

Since the [OH ^- ] ≈ [the HS ^- ] is

[H ₂ S]=Kh2 ^Θ =9.