For NH3, Kb = 1.8x10-5. You prepare a mixture of 15.0 mL 0.010 M NH3 5.0 mL 0.0100 M NH4Cl. You then add 0.50 mL 0.0100 M HCl. What is the pH of the final mixture? Suggestion: Start by figuring out the pKa of NH4 .