Please help with this problem

Question

Suppose you want to prepare a buffet of pH = 5.5 of the pair HCN/NaCN. calculate the mol ration NaCN/HCN necessary to prepare 1 liter of the buffer. Ka HCN= 4.0 x 10^-10

Answer 1

It is probably easiest to use the Henderson Hasselbalch equation for this problem (although you could use equilibrium expression).

Henderson Hasselbalch equaiton:

pH = pKa + log [salt]/[acid]

pKa = -log Ka = -log 4.0x10^-10 = 9.40

5.5 = 9.40 + log [salt]/[acid]

Solving for [salt]/[acid] we have ...

log [salt]/[acid] = -3.9

[salt]/[acid] = 0.00126 : 1 = mol ratio of NaCN : HCN

(NOTE: This would not be a very effective buffer at pH 5.5 since the pKa is so far from 5.5)