What happens to the strength of a base as its Ka value decreases?

The strength of a base is inversely related to its acid dissociation constant, Ka, which applies when considering the conjugate acid of the base in question. As the Ka value decreases, it indicates that the conjugate acid has a weaker tendency to donate protons, which means that the base itself is not effectively accepting protons in solution. This weaker ability to accept protons translates to a weaker base.

In other words, a lower Ka value means the equilibrium of the dissociation of the conjugate acid lies further to the left, leading to less of the base's corresponding anion being formed. This results in diminished basicity, making the base weaker overall.

Thus, a decrease in the Ka value of the conjugate acid signifies a weaker associated base, affirming why the correct response is that the base becomes weaker as its Ka value decreases.