Jigsaw activities for Day 2 – groups of 4 (so 6 groups and 7 groups in AP2/AP4)
Group 1, 4, 7, 10, 13
Goal: Make a buffer of desired pH
How many moles of potassium acetate must be added to 0.50 L of 0.35 M acetic acid (Ka =
1.8 X 10-5) in order to prepare a buffer of pH 5.08?
How many moles of potassium lactate must be added to 0.75 L of 0.75 M lactic acid (Ka =
1.4 X 10-4) in order to prepare a buffer of pH 3.38?
For buffers you may always use the HH equation. The 5% rule always holds for a buffer. If it doesn’t hold
then you don’t have much of a buffer.
Jigsaw activities for Day 2 – groups of 4 (so 6 groups and 7 groups in AP2/AP4)
Group 2, 5, 8, 11
Goal: Determine the pH of a buffer
What is the pH of a buffer that is 0.55 M in acetic acid and 0.35 M in sodium acetate? For
acetic acid Ka = 1.8 X 10-5.
What is the pH of a buffer that is 0.25 M in lactic acid and 0.65 M in sodium lactate? For
lactic acid Ka = 1.4 X 10-4.
For buffers you may always use the HH equation. The 5% rule always holds for a buffer. If it doesn’t hold
then you don’t have much of a buffer.
Jigsaw activities for Day 2 – groups of 4 (so 6 groups and 7 groups in AP2/AP4)
Group 3, 6, 9, 12
Goal: Determine the pH after diluting a buffer
A buffer is 0.55 M in acetic acid and 0.35 M in sodium acetate. This buffer has pH 4.55.
100 mL of this buffer is diluted to 160 mL. What is the pH after dilution? For acetic acid Ka
= 1.8 X 10-5.
A buffer is 0.75 M in lactic acid and 0.25 M in sodium lactate. This buffer has pH 3.38.
34 mL of this buffer is diluted to 50 mL. What is the pH after dilution? For lactic acid Ka =
1.4 X 10-4.
For buffers you may always use the HH equation. The 5% rule always holds for a buffer. If it doesn’t hold
then you don’t have much of a buffer.
Home Groups
Buffer Jigsaw Activity
Each person should spend 2 minutes explaining the question she was asked to solve and
how her ‘expert’ group solved it. Share any major conclusions.
Next work as a group to solve the following problems:
1. Determine the pH of a solution formed by adding 15 g of benzoic acid and 10 g of sodium
benzoate to water to form 1.0 L of solution. Benzoic acid (C6H5CO2H) has pKa = 4.2.
2. Calculate the mass of sodium acetate that must be added to 500.0 mL of 0.200 M acetic
acid to form a pH = 5.00 buffer solution.
Consider a solution containing both lactic acid (Ka = 1.4 X 10-4) and a lactate salt. Calculate
the ratio [lactate]/[lactic acid] if the solution has
a. pH = 3.25
b. pH = 3.55
c. pH = 3.85
d. pH = 4.15
e. pH = 4.45
For buffers you may always use the HH equation. The 5% rule always holds for a buffer. If it doesn’t hold
then you don’t have much of a buffer.