what is the importance of measuring the pH of a solution in spectrophotometric analysis for pKa?

Answer 1

For weak mono-hydrogen acids (HM), if the amount of weak acid added to water is m moles,
[H+][M-]/m = Ka. The amount of [H+] and [M-] are the same, so [H+]^2= mKa.
Since pH relates to [H+], knowing it and m will provide the value of Ka, and hence pKa.