Acids and Bases (HL) - Lesson 9 Buffers

Refresh Determine the Kb and pKb of the ethanoate ion. Calculate the pH and pOH of 0.200 mol dm–3 ethanoic acid (pKa = 4.76). Determine the Kb and pKb of the ethanoate ion.

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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.

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 7.35-7.45

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

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

Calculating pH of a Buffer What is the pH of a buffer comprising a 0.450 mol dm-3 ammonia (NH3, Kb = 1.8x10-5) and 0.200 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

Calculating the proportions for a buffer What mass of sodium ethanoate (Mr = 82.0) should be added to 0.500 dm3 of 0.400 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

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

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