Question

If the concentration of KCN at the equivalence point is 0.1M and the Ka for HCN is 6.2*10^-10 find the pH at the equivalence point.

The reaction is between KOH and HCN
From my understanding I have to do a stoichiometric calculation and then use the Henderson-Hasselback equation to find pH. Is this correct?

Is the equation for the stoichiometry as follows:
HCN+ H2O --> H30+ + CN- ????
Does the 0.1M concentration of KCN represent the end point of the CN- concentration?

Is it possible to calculate the pH with only the limited information?

Answer 1

I believe you're on the wrong track.
This is a hydrolysis equation; i.e., at the equivalence point of an acid/base titration, you are left with the salt and water. The salt hydrolyzes (unless it is the salt of a strong base/strong acid such as NaCl, KCl, KBr etc).
......CN^- + HOH ==> HCN + OH^-
I.....0.1.............0.....0
C.....-x..............x.....x
E....0.1-x............x.....x

Kb of CN^- = (Kw/Ka of HCN)= (x)(x)/(0.1-x)
Solve for x = (OH^-). Convert to pH.