Question

Determine the equilibrium pH for .0001 M AmmoniumChloride, NH4Cl (salt of the weak conjugate acid,pK=9.25).

How much hypochlorous acid (HOCl; pKa=7.60) must beadded to pure water to make solution of pH 4.3? pH6.5?

The method my instructor uses is to set up equations forequilibria, mass balance, and charge balance. Then makingassumptions, solve for [H+] using a system of equations but I'm notsure about the assumptions. If some one could answer thesequestions/explain this method I would greatly appreciate it!