Question

Consider a solution made by mixing HCN (Ka = 6.2 × 10–10) with HC2H3O2 (Ka = 1.8 × 10–5) in aqueous solution. What are the major species in solution? H+, CN–, H+, C2H3O2–, OH–, H2O

H+, CN–, HC2H3O2, H2O

HCN, HC2H3O2, H2O

H+, CN–, H+, C2H3O2–, H2O

HCN, H+, C2H3O2–, H2O

Please explain with your answer.

Answer 1

HCN will dissociate partially to give

HCN(aq) H⁺(aq) + CN^-(aq)

The Ka of this is 6.2 x 10^-10

so it is very little dissociated and the amount of CN^- is assuming a 1M solution 2.5 x 10^-5 M

HC₂H₃O₂ will dissociate to give

HC₂H₃O₂ (aq) H+(aq) + C₂H₃O₂^-(aq)

The Ka for this is 1.8 x 10^-5 and the amount of C₂H₃O₂^- is assuming a 1M solution 4.24 x 10^-3 M.

Thus C₂H₃O₂^- is about 500 times less than HC₂H₃O₂ and C₂H₃O₂^- is about 100 times more than the CN^-

So the major species in solution are

HCN, HC₂H₃O₂, H2O