What is the relationship between pKa and Ka?

The relationship between pKa and Ka lies in the definition of pKa, which is derived from the acid dissociation constant (Ka) of a given acid. Specifically, pKa is defined as the negative logarithm (base 10) of the Ka value:

[ \text{pKa} = -\log(\text{Ka}) ]

This relationship indicates that as the Ka value increases, representing a stronger acid that dissociates more completely in solution, the pKa value decreases. Therefore, pKa provides an intuitive way to express the strength of an acid: a lower pKa signifies a stronger acid because it correlates to a higher Ka.

The correct choice highlights this fundamental relationship, emphasizing the logarithmic relationship between the two values. Understanding this allows for better insight into acid-base equilibria, making it easier to compare the strengths of different acids.