Final answer:
A solution with the equilibrium constant Kb = 1.8x10-5 is classified as a weak base.
Step-by-step explanation:
The equilibrium constant Kb is used to measure the relative strength of bases in an aqueous solution. A solution with Kb = 1.8x10-5 is classified as a weak base. This means that it ionizes only slightly in water, resulting in a lower concentration of hydroxide ions compared to a strong base.