1. Acids and Bases (HL) - Lesson 9
Buffers

2. Refresh Determine the Kb and pKb of the ethanoate ion.
Calculate the pH and pOH of mol dm–3 ethanoic acid (pKa = 4.76).
Determine the Kb and pKb of the ethanoate ion.

3. We Are Here

4. Lesson 8: Buffers Objectives:
Describe and explain the function of buffers
Calculate the pH of buffer solutions
Make an acid and an alkaline buffer solution and observe its buffering activity.

5. Buffers Buffers are solutions that resist changes to their pH
Acid buffers:
Weak acid and salt of it’s conjugate base
E.g. Ethanoic acid and sodium ethanoate
Alkaline buffer:
Weak base and a salt of its conjugate acid
E.g. ammonia and ammonium chloride
Common in nature, blood being the best example: pH kept at

6. How they work: acidic buffers
Buffer is a mixture of HA and A- solutions, establishing the following equilibrium:
HA  H+ + A-
The addition of extra A- ions forces the equilibrium to the left due to Le Chatelier’s principle. This creates a large reservoir of un-dissociated HA
Adding acid:
Equilibrium shifts to the left, reducing the increase in H+
Adding base:
Equilibrium shifts to the right, reversing the decrease in H+
Adding water:
Both sides affected equally so pH unchanged

7. How they work: basic buffers
Buffer is a mixture of BOH and B+ solutions, establishing the following equilibrium:
BOH  B+ + OH-
The addition of extra B+ ions forces the equilibrium the left due to Le Chatelier’s principle. This creates a large reservoir of un-dissociated BOH
Adding base:
Equilibrium shifts to the left, reducing the increase in OH-
Adding acid:
Equilibrium shifts to the right, reversing the decrease in OH-
Adding water:
Both sides affected equally so pH unchanged

8. Calculating pH of a Buffer
What is the pH of a buffer comprising a mol dm-3 ammonia (NH3, Kb = 1.8x10-5) and mol dm-3 ammonium chloride?
We need to make two assumptions:
[NH4+] is equal to that stated in the question…since there will be very little coming from dissociated NH3
[NH3] is equal to that stated in the question…since very little NH3 has dissociated

9. Calculating the proportions for a buffer
What mass of sodium ethanoate (Mr = 82.0) should be added to dm3 of mol dm-3 ethanoic acid solution (pKa = 4.76) to make a buffer of ph 5.70?
Similar to before:
Two assumptions:
CH3COO- will be only due to added sodium ethanoate
[CH3COOH] is as stated in the question
Note: Ka= 10-pKa
Mass sodium ethanoate required:
Mass = moles x molar mass = (3.48 x 0.500) x 82.0 = 143 g

10. Making Buffers
You will need to follow instructions to make an acid buffer, and then adapt those instructions to make an alkaline buffer of pH 10
Once made, the buffers will be tested to show their buffering activity
Follow the on the instruction sheet

11. Key Points
Calculations are similar to those for weak acids/bases but need re-expressing as:
Calculations make two key assumptions:
Concentration of the weak acid/base is as stated in the question
Concentration of the conjugate base/acid is as stated in the question
These assumptions are valid as equilibrium effects mean very little of the weak acid/base dissociates