Chem 112 Class Guide: ACID/BASE TITRATIONS (Chapter 17, Section 3)
Chapter 17, Section 3 Learning Goals:
Upon completion of Chapter 16, Sections 6-9, you should be able to determine the following:
Identify the equivalence point in a titration
Recognize whether a titration curve is strong acid/strong base or weak acid/strong base
Calculate the pH at any point in a strong acid/strong base titration
Calculate the pH at the equivalence point of a weak acid/strong base titration
Calculate the pH at the equivalence point of a strong acid/weak base titration
Chapter reading Guide: Chapter 17, Section 3
Section 3: ACID/BASE TITRATIONS
Read Chapter 17.3
The above plot is an example of a titration curve obtained from an acid/base titration. The
curve can be broken down into 4 distinct areas:
1) Black dot – initial pH
2) Between black and blue dots – pre-equivalence point
3) Blue dot – equivalence point
4) After the blue dot – post-equivalence point
For a strong acid/strong base titration, the pH of the solution is calculated as follows:
1) Initial pH – as you did in Chapter 16 for a strong acid
2) Pre-equivalence point – the pH is based on the amount of acid that has not yet reacted
3) Equivalence point – pH = 7
4) Post-equivalence point – the pH is based upon the amount of excess base
For a weak acid/strong base titration, the pH of the solution is calculated as follows:
1) Initial pH – as you did in chapter 16 for a weak acid
2) Pre-equivalence point – you are not responsible for this
3) Equivalence point – described below
4) Post-equivalence point – the pH is based upon the amount of excess base
For the equivalence point of a weak acid/strong base titration, you know that the pH will be
greater than 7 (because we have a STRONG base, but a WEAK acid), but what is the pH? In
order to figure that out, you must know that the reaction that occurs is a 3-step process,
with the last step being the step that determines the pH.
For a weak acid (HA) and a strong base (BOH):
HA  BOH  BA  H 2O
BA  B   A
A  H 2O  HA  OH 
The last step is the equilibrium.
You can calculate the number of moles of acid, but its molarity will be based upon the total
volume (including base). Set up an ICE chart, solve for X…..but keep in mind that you are
likely to be given the Ka of the acid. Since the equilibrium contains OH-, you will need to
convert Ka to Kb before solving for x. Solve for x, calculate pOH and then pH.
Example:
What is the pH when 50.0 mL of 1.50 M HC2H3O2 is titrated with 50.0 mL of 1.50 M NaOH?
The Ka of HC2H3O2 is 1.8x10-5.
HC2H3O2 + NaOH  NaC2H3O2 + H2O
NaC2H3O2  Na+ + C2H3O2C2H3O2- + H2O ↔ HC2H3O2 + OHThe last equation is the important one. We will need the Kb of C2H3O2-, which we can
calculate using Kw: K a * Kb  K w
Kb 
K w 1.00 x1014

 5.56 x1010
5
Ka
1.8 x10
We also need the concentration of C2H3O2- at equilibrium (remember that the solution has
been diluted by the amount of base we added). The total volume of the solution is 100.0 mL
(50.0 mL acetic acid + 50.0 mL sodium hydroxide).
M1V1 = M2V2
M2 
M1V1 1.50M  50.0mL 

 0.75 M
V2
100.0mL
Now we are ready to do an ICE chart:
Initial
Change
Equilibrium
Kb
C2H3O20.75 M
-X
0.75 – X
 HC2 H 3O2  OH    X  X 


C2 H 3O2  
OH0
+X
X
HC2H3O2
0
+X
X
0.75  X

X2
 5.6 x105
0.75
So X = 2.0x10-5 = [OH-]
pOH = -log[OH-] = -log(2.0x10-5) = 4.7\
pH + pOH = 14
pH = 14 – pOH
pH = 14 – 4.7 = 9.3
Try Practice exercises 17.6 and 17.8a
Learning Resources
Chapter Learning Goals
Chapter 17, Section 3 Learning Goals
Pre Class Assignment: This assignment must be completed prior to the next class. Check your syllabus
for the exact due date and time.
Complete the pre class assignment
(http://www.bk.psu.edu/clt/chem112/Acid-BaseTitrations_HW.docx)
Submit a copy to the dropbox located in ANGEL called “Pre Class Assignment Submission: Acid /
Base Titrations”
End of Chapter Problems: Chapter 17, Section 3
Practice with these problems if you are having difficulty with any of the concepts covered in this
class guide AFTER we have met in class. If you cannot easily complete these problems, seek
help from your instructor, your mentor or the learning center
Chapter 17: 37, 41, 45a, 45d