What is the pH of a solution made by adding 50 mL 0.1 M HCl to 150 mL 0.099 M NH3?
50 mL x 0.1M = 5 mmoles HCl.
150 mL X 0.099 M = 14.85 mmoles NH3.
.................NH3 + HCl ==> NH4Cl
initial.........14.85..5.0......0..
change..........-5.0....-5.0....+5.0
equil...........9.85.....0......5.0
Substitute into the Henderson-Hasselbalch equation and solve for pH.