Question

what is the ph of a solution that is .15 M in HNO3 and 2.0 M HCN?

Answer 1

HCN is very weak acid and HNO3 is very strong acid. Due to the common ion effect in presence of strong acid dissociation of weak acid is supressed.

So in presence of HNO3 dissociation of HCN is supressed. That means dissociation of HCN is very very low, hence it can be negligible.

Hence the concentration of H+ ion is only from HNO3

HNO3 (aq) + H2O(l) -----------> H3O+ (aq) + NO3- (aq)

[H3O+] = [HNO3]

= 0.15 M

pH = -log[H3O+]

   = -log(0.15)

= 0.82

Hence pH of the solution = 0.82