Consider 1.00 L of the buffer system that contains 0.200 M hydrocyanic acid(HCN) and 0.150 M sodium cyanide (NaCN). The pKa of hydrocyanic acid is 9.31.

Question

What is the [HCN] after 0.020 mol of HCl is added?

Answer 1

Add 0.02 mol HCl to 1L means (H^+) = 0.02M

.......CN^- + H^+ ==> HCN
I....0.150....0.......0.2
add.........0.02.........
C...-0.02..-0.02......+0.02
E....0.130...0.......+0.22

(HCN) = 0.22M assuming the HCl was added as a gas

Answer 2

0.200m+0.20=0.22M correct