How do you find the pH of salt?

Find the pH of a solution of ammonia and HBr. I don't understand how to find this pH! Please help!?

• The full question is: 574.9 mL of a 0.12 M solution of ammonia is mixed with 574.9 mL of a 0.12 M solution of HBr. What is the pH of the resulting solution of NH4Br? (Kb for NH3 equals 1.8 × 10-5) I know that means that both start with 0.068988 moles of themselves, but when I draw an ICE diagram I end up with 0 moles remaining of NH3 and HBr. Maybe my equation is off because I didn't add water to it. I think it should be NH3 + HBr --> NH4Br. I'm confused how to find the pH because there aren't any OH- or H3O+ ions floating around. Please help!

• Answer:

  NH3 + HBr -> NH4Br. Since both are 0.068988 moles, same amount of NH4Br will be produced. But there is also water in the solution as both are aqueous solution. NH4Br will dissociate into NH^4+ and Br-. Br- is a weak conjugate base of strong acid HBr. So it won;t affect the pH. NH^4+ is a RELATIVELY strong conjugate acid of weak base NH3. Therefore, it will setup an equilibrium with water. NH^4+ + H2O <=> NH3 + H3O+ You have the moles of NH^4+ (same as NH4Br), and the total volume is (574.9 mL x 2). Find the concentration of NH^4+ and setup an ICE table. You will get the [H3O+] from which you can get the pH. Note: Ka for NH^4+ is (1 x 10^-14)/ Kb for NH3.