An aqueous solution contains 0.30 M potassium cyanide. One liter of this solution could be converted into a buffer by the addition of:

Hydrocyanic acid, HCN.

Explanation:

A buffer is defined as the aqueous mixture of a weak acid and its conjugate base. Or a weak base and its conjugate acid. In the problem, we have a solution of potassium cyanide. The conjugate acid of potassium cyanide is hydrocyanic acid. A weak acid with pKa = 9.89.

That means, to convert the solution into a buffer we must add:

they both have a y in them