Ionic Equilibrium

The ion product of water (Kw) is the product of the acid dissociation constant (Ka) and the base dissociation constant (Kb) :
Ka×Kb=Kw
So, you can solve for Kb using:
Kb=‌

Kw

Ka

We know:
‌Kw=1.0×10−14‌ at ‌25∘C
‌Ka=1.8×10−4
Plug these values into the formula:
‌Kb=‌

1.0×10−14

1.8×10−4

‌Kb=5.55×10−11