Acid and base equilibrium

Question

OS Chapter 14 Acids and Bases Buffers and Titrations Items Determine the ammonia concentration of an aqueous solution that has a pH of 11. 00. The pKb of NH3 is 4.75. Show work.

Answer 1

First, write the balanced equation:

NH₃ + H₂O ==> NH₄⁺ + OH^-

From the pH = 11.00, we can find the [H⁺] and then the [OH^-] which we will use in the Kb expression.

From pH = 11, [H⁺] = 1x10^-11

[H⁺][OH^-] = 1x10^-14

[OH^-] = 1x10-¹⁴/1x10^-11 = 1x10^-3 M

Kb = [NH₄⁺][OH^-]/[NH₃] and from pKb = 4.75, Kb = 1x10^-4.75 = 1.78x10^-5

1.78x10^-5 = (1x10^-3)(1x10^-3)/[NH³]

[NH₃] = 0.0562 M