What characteristic defines the weakest base when comparing based on Ka?

The weakest base is defined by having the smallest Ka value. Ka, which stands for the acid dissociation constant, measures the strength of an acid within a solution; however, it can also provide insights about the strength of a base through the relationship between acids and their conjugate bases. In this context, the base strength is often inversely related to its conjugate acid's Ka.

When one compares the options, the choice of the smallest Ka value is pivotal because a lower Ka indicates that the acid does not dissociate significantly in solution, demonstrating that its conjugate base (which is the species we are concerned with when discussing base strength) is weak. Therefore, the base that corresponds to the smallest Ka is indeed the weakest base since it indicates minimal proton affinity.

In the context of the other options, mention of the exponent is somewhat misleading, as the emphasis should solely be on the value of Ka. The presence of a "highest exponent" or "lowest exponent" isn't a direct way to measure base strength; rather, higher exponents could imply that you are dealing with a strong concentration of an acid or base, which does not contribute to the correct identification of weakness based on Ka values alone. The ideal focus is on selecting the smallest numerical Ka