Question

Given that Ka for HCN is 6.2*10^-10 at 25°c, what is the value of Kb for CN- at 25°c?

Given that Kb for NH3 is 1.8*10^-5 at 25°c, what is the value of Ka for NH4+ at 25°c?

Answer 1

We know that K_a x K_b = K_w = 1.0x10^-14

(1) Given K_a for HCN is 6.2x10^-10

             K_b for CN^- is = K_w / K_a

                                = (1.0x10^-14) / (6.2x10^-10)

                                = 1.61x10^-5

(2) Given K_b for NH₃ is 1.8x10^-5

             K_a for NH₄⁺ is = K_w / K_b

                                = (1.0x10^-14) / (1.8x10^-5)

                                = 5.55x10^-10