One student measure the pH of the following three solutions (weak base, conjugat

Question

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

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