Posted by Sharon on Monday, July 18, 2011 at 10:04pm.
http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html
Here are some titration curves. Scroll down to the weak base versus strong acid (there is a curve there where the acid is run INTO the base). Divide the curve up into 4 regions.
A. beginning point.
C. equivalence point.
B. all points between A and B.
D. All points after C.
A. You have a pure weak base. Make an ICE chart, substitute into Kb expression and solve for OH^- and convert to pH.
C. The equivalence point is determined by the salt produced. The equation is
NH3 + HCl ==> NH4Cl
The NH4^+ acts as a Bronsted-Lowry acid and hydrolyzes. Write the hydrolysis equation and make an ICE chart for that, substitute into Kb expression, and solve for H^+, then convert to pH. Many students forget to take into account the dilution of the salt but you need to do that, too.
B. All points between A and B use the Henderson-Hasselbalch equation.
D. All points AFTER the equivalence point are determined by the amount of excess HCl which is a strong acid (it ionizes 100%).
The first think you should do is determine how many mL it takes to reach the equivalence point so you can properly divide the curve into the proper regions.