Question

A) In the laboratory, a general chemistry student measured the pH of a 0.533 M aqueous solution of phenol (a weak acid), C₆H₅OH to be 5.154.

Use the information she obtained to determine the K_a for this acid.  

K_a(experiment) = _____________

B) In the laboratory, a general chemistry student measured the pH of a 0.533 M aqueous solution of hypochlorous acid to be 3.881.

Use the information she obtained to determine the K_a for this acid.  

K_a(experiment) = _________

Answer 1

A) C₆H₅OH(aq) <----------> C₆H₅O^-(aq) + H⁺(aq)

Now, pH = -log[H⁺]

Thus, [H⁺] = 10^-pH = 10^-5.154 = 7.015*10^-6 M

Now, at eqb., [C₆H₅OH] = (0.553 - (7.015*10^-6)) M & [C₆H₅O^-] = [H⁺] = (7.015*10^-6) M

Thus, K_a = {[H⁺]* [C₆H₅O^-]}/[C₆H₅OH] = 9.232*10^-11

B) HClO(aq) <-----------> H⁺(aq) + ClO^-(aq)

Now, pH = -log[H⁺]

Thus, [H⁺] = 10^-pH = 10^-3.881 = 1.315*10^-4 M

Now, at eqb., [C₆H₅OH] = (0.553 - (1.315*10^-4)) M & [C₆H₅O^-] = [H⁺] = (1.315*10^-4) M

Thus, K_a = {[H⁺]* [ClO^-]}/[HClO] = 3.245*10^-8