Answer:
Kb = [CH₃NH₃⁺] [OH⁻] / [CH₃NH₂]
Step-by-step explanation:
In this case, to write the equilibrium expression you first need to write the overall reaction.
For start, Kb means basic constant in equilibrium of a aqueous solution. This os often used in acid base reactions.
So, to write the overall reaction, you need to write an acid base reaction between the base and water, and apply the bronsted lowry theory or the lewis theory for acid base reactions.
Knowing this, let's write the overall reaction
CH₃NH₂ + H₂O
In this case, the CH₃NH₂ would be the base (Because we want it's Kb) and water would act as acid. So, the water will donate an Hydrogen, and the base will accept that (Bronsted lowry theory)
CH₃NH₂ + H₂O -------> CH₃NH₃⁺ + OH⁻
Now that we have the overall reaction we can write the Kb expression. This would be [Products] / [Reactants]. However we only take the reagents in aqueous or gaseous state. Liquid and solid do not contribute to the equilibrium expression. In this case, only water is liquid. The rest is aqueous so the Kb expression would be:
Kb = [CH₃NH₃⁺] [OH⁻] / [CH₃NH₂]