116 PLTL Activity sheet / Titrations
Set 10
Potentially Useful or Useless Information: Definitions
Titration Curve ⇒ The plot of pH of solution vs. volume of titrant (acid or base) added
♦ Shape of acid-base titration curve gives information on what is titrated
Equivalence Point ≡ the point in a titration where stoichiometric amounts of both reagents are present
♦ i.e. amt of added base just exactly neutralizes the initial amount of acid
End Point ≡ point in the titration (pH) where the indicator changes color
♦ Choose indicator so that equivalence point coincides with the pH of the endpoint
Indicator ≡ substance that exhibited color in solution depends upon the pH
♦ Usually a weak organic acid or weak organic base
Titration of Strong Acid with Strong Base
♦ Product is a neutral salt and water
♦ pH at equivalence point is neutral
[pH
of exactly 7 at 25°C]
Stage
I
Prior to the addition of base
II Prior to the equivalence
point
III At the equivalence point
IV After the equivalence point
pH of solution
Acidic-solve as strong acid problem
Acidic, although some of the acid has been neutralized
pH < 7
pH < 7
Aqueous neutral salt solution
Basic solve as strong base problem
pH = 7
pH > 7
Activity 1: Consider the titration of 50.00 mL 0.2000 M HCl titrated with 0.2000 M NaOH
Write the neutralization reaction
In titration of any strong acid by any strong base, what type of salt is present at the equivalence point?
Stage
I
How many millimoles of acid are present?
1
116 PLTL Activity sheet / Titrations
Set 10
What is the pH of the titration solution?
II
Halfway to the equivalence point
How many millimoles of base have been added?
How many millimoles of acid are remain?
What is the total volume of the titration solution?
What is the pH of the titration solution?
III
At the equivalence point
How many millimoles of base have been added?
What is present in solution [in terms of ions, acids, bases etc]?
What is the total volume of the titration solution?
What is the pH of the titration solution?
IV
After Equivalence point (let’s say 25 mL of excess base added)
How many millimoles of base have been added?
What is present in solution [in terms of ions, acids, bases etc]?
What is the total volume of the titration solution?
What is the pH of the titration solution?
2
116 PLTL Activity sheet / Titrations
Set 10
Activity 2
Titration of Weak Acid with Strong Base
♦ Product is a basic salt and water
♦ pH at equivalence point is basic
Stage
I
Prior to the addition of base
II Prior to the equivalence point
III At the equivalence point
IV After the equivalence point
pH of solution
Acidic—solve as weak acid problem
Buffer solution
Basic Salt: Weak base problem
Strong base problem
Activity 2: Consider the titration of an acetic acid solution with sodium hydroxide solution
Write the neutralization reaction
Describe the composition of the solution at the following stages of the titration
Stage
I
Before the titration begins
II
When the number of moles of sodium hydroxide added is equal to half the number of moles of
acetic acid originally in the flask
III
When the number of moles of NaOH added is equal to the number of moles of acetic acid
originally in the flask (the equivalence point
3
116 PLTL Activity sheet / Titrations
Set 10
Activity 3
Consider the titration of 25.0 mL 0.200 M HC2H3O2 (KA = 1.8×10−5) titrated with 0.200 M NaOH.
Write the neutralization reaction
Determine the volume NaOH (in mL) needed to reach the equivalence point
For each volume given below list the major species in solution and determine the Ka equation, Kb
equation, the buffer equation, or solution equilibrium equation is appropriate for the calculation of [H+]
and pH. Complete the calculations below and on the back of the sheet.
NaOH added
0.00 mL
Major species
Appropriate equation
[H+]
pH
10.00 mL
25.00 mL
40.00 mL
50.00 mL
60.00 mL
4