J.R. S. answered 03/24/21
Ph.D. University Professor with 10+ years Tutoring Experience
Use the Henderson Hasselbalch equation pH = pKa + log [salt]/[acid]
Solve for [salt]/[acid]...
pKa = -log Ka = 9.31
9.50 = 9.31 + log [salt]/[acid]
log [salt]/[acid] = 0.19
[salt]/[acid] = 1.55
So you need 1.55 : 1 for the ratio of NaCN to HCN