Ammonia acts as a base in aqueous solution

[a.a]

Homework Statement

Ammonia acts as a base in aqueous solution; Kb is equal to 1.8 x 10-5. What is the Ka for the ammonium ion NH4+ in aqueous solution?

Homework Equations

The Attempt at a Solution

I am totally lost for this question, can someone please start me off? all i know is that Kw = Ka * Kb

 

[Borek]

all i know is that Kw = Ka * Kb

You are given Kb, you should remember Kw value - just solve for Ka.