What is the formula used to calculate pKa?

The formula to calculate pKa is derived from the acid dissociation constant (Ka), which quantifies the strength of an acid in solution. Specifically, the relationship is defined as pKa being the negative logarithm of the Ka value. This is expressed mathematically as pKa = -log(Ka).

The pKa value is particularly useful in chemistry because it allows for easy comparison of acid strengths, with lower pKa values indicating stronger acids. For instance, if the Ka of a strong acid is significantly greater than 1, the pKa will be a negative value, emphasizing the strength of the acid.

Using the logarithmic scale allows chemists to work with manageable numbers, as the values of Ka can vary over many orders of magnitude. Therefore, understanding this relationship is vital for anyone studying aqueous chemistry and acid-base equilibria.

The other options do not provide the correct relationship between pKa and Ka as they either give a direct equality, which is not valid, or introduce incorrect mathematical operations that do not reflect the logarithmic relationship needed to find pKa.