Question

To find the pKa of X-281, you prepare a 0.075 M test solution of X-281 at 25.0 ∘C. The pH of the solution is determined to be 3.00. What is the pKa of X-281?

Answer 1

X-281 <--------------------> X-281-    +    H+

0.075                                     0                0

0.075 - x                                x                 x

Ka = [X-281-][H+] / [X-281]

[X-281] = 0.074 M

pH = 3.00

[H+] = 10^-3.00 = 0.001 M = [X-281-]

Ka = (0.001)^2 / 0.074 = 1.33 x 10^-4

pKa = -log[Ka] = 4.87