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 [CN-] after 0.020 mol of HCl is added?

Answer 1

I work these problems with millimoles.
mmols HCN = mL x M = 1000 mL x 0.200 M = 200
mmols NaCN = 1000 mL x 0.150 = 150
.....................CN^- + H^+ ==> HCN
Initial.............150.....................200
add............................20.......................
change..........-20......-20............+20
equilibrium...130.........0..............220
(CN^-) = M = millimoles/mL = 130/1000 = ? assuming the HCl is added as a gas. If added as an aqueous solution, then total mL will be 1000 + ? mL added solution.
Post your work if you get stuck.