Asolution is prepared by dissolving 0.23 mol of hydrocyanic acid and 0.27 mol of sodium cyanide in water sufficient to yield 1.00 l of solution. the addition of 0.05 mol of hcl to this buffer solution causes the ph to drop slightly. the ph does not decrease drastically because the hcl reacts with the present in the buffer solution. the ka of hydrocyanic acid is 4.9 × 10-10.