At 298 K , the pK_a of hydrocyanic acid ( HCN ) is 9.30 and the...

Ammonium cyanide ($\text{NH}_4\text{CN}$) is a salt of a weak acid ($\text{HCN}$) and a weak base ($\text{NH}_3$).

The pH of an aqueous solution of a salt of a weak acid and a weak base is given by the formula:

$\text{pH} = 7 + \dfrac{1}{2}(pK_a - pK_b)$

Substituting the given values:

$\text{pH} = 7 + \dfrac{1}{2}(9.30 - 4.74)$

$\text{pH} = 7 + \dfrac{1}{2}(4.56)$

$\text{pH} = 7 + 2.28 = 9.28$

Answer: $9.28$