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.
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