We are asked to find Kb for CN-. CN- is the conjugate base of the weak acid HCN
HCN + H2O ⇌ H3O+ + CN-
(weak base) (acid) (conjugate acid) (conjugate base)
Hence, the Kb of CN- can be calculated from the Ka of HCN. Ka and Kb are related by the autoionization constant of water (Kw) by the following equation:
Given that Ka for HCN is 4.9x10-10 calculate Kb for CN-
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