Determine the degree of hydrolysis and pH of the potassium cyanide solution with a molar concentration of 0.005 mol / L.

Decision:
Potassium cyanide is a salt formed by the strong base KOH and the weak acid HCN, which is hydrolyzed by anion in an aqueous solution.
KCN + HOH ⇔ KOH + HCN
CN– + HOH ⇔ HCN + OH–
Thus, upon hydrolysis of this salt in the solution, there will be an excess of OH– ions, which gives the solution an alkaline reaction
(pH> 7 – alkaline medium).
The hydrolysis constant of a salt formed by a weak acid and a strong base is determined by the formula:
Kr = Kw / KD (acids)
Kw = 1. 10-14 – ionic product of water.
KD (HCN) = 5. 10-10 – the dissociation constant of cyanic acid
The potassium cyanide hydrolysis constant is:
Kr (KCN) = Kw / KD (HCN) = (1.10-14) / (5.10-10-10) = 2. 10-5.
Calculate the molar concentration of OH? in solution, we get:
[OH–] = h. CM (KCN) = (4.10-3). 0.005 = 2. 10-5 mol / l.
We find the hydroxyl index, we get:
pOH = – log [OH–] = – log2. 10-5 = 5 – log2 = 5 – 0.30 = 4.7.
We find the hydrogen indicator, we get
pH = 14 – pOH = 14 – 4.7 = 9.3.
Answer: pH = 9.3.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.