We are asked to find Ka for NH4+. NH4+ is the conjugate acid of the weak base NH3
NH3 + H2O ⇌ NH4+ + OH-
(weak base) (acid) (conjugate acid) (conjugate base)
Hence, the Ka of NH4+ can be calculated from the Kb of NH3. Ka and Kb are related by the autoionization constant of water (Kw) by the following equation:
Given that Kb for NH3 is 1.8x10-5 calculate Ka for NH4+
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