Salt of a Strong Base and a Weak Acid. pH of solution?

NH3 is a weak base (Kb = 1.8 × 10–5) and so the salt NH4Cl acts as a weak acid. What is the pH of a solution?

• I cant seem to get this. I have the Ka = 5.6x10^-10 but after that Im lost

• Answer:

  The ammonium ion of the salt does act as a weak acid by the equation: NH4+ + H2O <---> NH3 + H3O+ The equation for Ka is: Ka = [NH3+][H+]/[NH4+] = 5.6X10^-10 In the solution, [NH3] = [H+]. Let both of those = x. In order to calculate the pH, you will need to know the concentration of the ammonium chloride (specifically of the ammonium ion). With that, you just plug into the expression for Ka and solve for x, which you defined to be [H+]. From that you can calculate the pH as pH = - log [H+]