How to calculate pH of a solution containing salt of a weak base?

A salt of a weak base (B) is composed of the conjugate acid (BH⁺) of the base and a counter ion which is negatively charged (e.g. chloride, Cl^- ). For example, ammonium chloride (NH₄Cl) is a salt of the weak base ammonia. When ammonium chloride dissolves in solution it completely ionize into ammonium ions (NH₄⁺) and chloride ions (Cl^-). Thus if we have 0.001 M solution of ammonium chloride then the solution contains 0.001 M ammonium ions and 0.001 M chloride ions. Ammonium ions in solution act as a weak acid (can donate a proton) according to the following chemical equation:

                                                              BH⁺ + H₂O ↔ H₃O⁺ + B

To calculate the pH of such a salt we need to determine Ka value of the conjugate acid (ammonium ions in the above example). Ka value can be calculated from Kb value of the base (see the post on making this calculation). After Ka is calculated and by knowing the concentration we can apply the equation for calculating pH of weak acids (see this post) or using online pH calculator for weak acids. For the above example, C = 0.001 M, Ka = 5.75 × 10^-10 , and pH = 6.12