One student measure the pH of the following three solutions (weak base, conjugate acid and buffer). Using the data, please calculate the pKa and pKb (Must show calculation) (a) pH = 8.7 for 0.010 M RNH2 (weak base): pKb = __________________________ (b) pH = 9.3 for 0.050 M NaA (conjugate base): pKa = ________________________ (c) pH = 4.5 for a buffer of 0.20 M acid HA and 0.10 M NaA (conjugate base) pKa = ______________________; pKb = ___________________________ Must show calculations. I need help step by step.
Explanation / Answer
b. pH = 9.3 for 0.050 M NaA -> Na+ + A- A- + H2O HA + OH- 0.050 0 0 -x +x +x 0.050-x x x x = [OH-] = 10^-(14-pH) = 2.00 x 10^-5 Kb = [HA][OH-]/[A-] = x^2/(0.050-x) = 7.97 x 10^-9 pKb = -logKb = 8.10 pKa = pKw - pKb = 14 - 8.10 = 5.90