Buffer solutionsEdexcel A-Level Chemistry: Revision notes
Section 1
What is a buffer?
A buffer solution resists changes in pH when small amounts of acid or alkali are added (and on moderate dilution). It contains a weak acid HA and its conjugate base A⁻ in significant amounts, with a large reservoir of each. HA ⇌ H⁺ + A⁻ provides both: HA is the weak acid and A⁻ comes mainly from a dissolved salt.
Two ways to make an acidic buffer: mix the weak acid with its salt (ethanoic acid with sodium ethanoate), or partially neutralise the weak acid with strong alkali, leaving excess acid.
A buffer does not keep pH exactly fixed and does not have pH 7. It resists change only for small additions.
Section 2
How a buffer works
Consider CH₃COOH ⇌ CH₃COO⁻ + H⁺ with large amounts of both species.
- Adding H⁺: reacts with the base, CH₃COO⁻ + H⁺ → CH₃COOH; the equilibrium shifts left.
- Adding OH⁻: reacts with the acid, CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O.
Either way the ratio changes only slightly, so and the pH are almost unchanged.
Name the species that reacts and write the equation.
Section 3
Calculating the pH of a buffer
Rearranging gives:
, so pH = −lg .
Worked example: 0.250 mol dm⁻³ ethanoic acid with 0.150 mol dm⁻³ sodium ethanoate: mol dm⁻³, so pH = 4.54. When , pH = pKₐ.
If a strong acid or alkali is added, first work out the amounts of HA and A⁻ after reaction. Because both are in the same volume, the ratio of amounts can be used.
Using for a buffer. That equation assumes , which is false when salt is present.
Section 4
Preparing a buffer of a given pH
To make a buffer of pH 5.00 from ethanoic acid: = 1.00 × 10⁻⁵ mol dm⁻³, so . If 100 cm³ of 0.500 mol dm⁻³ acid (0.0500 mol) is used, you need 1.74 × 0.0500 = 0.0870 mol of ethanoate, which is 174 cm³ of 0.500 mol dm⁻³ sodium ethanoate.
Choose an acid with pKₐ close to the required pH, so that the ratio is near 1 and the capacity is greatest.
Section 5
Buffers in blood
Blood is held at pH 7.35–7.45 by H₂CO₃ and HCO₃⁻, with CO₂(aq) in equilibrium: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. Extra H⁺ (e.g. lactic acid) reacts with HCO₃⁻ to form H₂CO₃, which decomposes to CO₂ that is exhaled. Extra OH⁻ reacts with H₂CO₃ to form HCO₃⁻ and water. The ratio of hydrogencarbonate to carbonic acid is about 20:1, giving more capacity for neutralising acid.
Link the buffer to breathing: removing CO₂ pulls the equilibrium and restores the ratio.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Buffer solutions
- A food scientist wants to keep the pH of a sauce steady during storage. She uses a mixture of ethanoic acid, CH₃COOH, and sodium ethanoate, CH₃COONa, in water. For ethanoic acid, = 1.74 × 10⁻⁵ mol dm⁻³ at 298 K.Explain how this mixture resists a change in pH when a small amount of hydrochloric acid is added.2 marks
- A technician prepares a buffer solution by dissolving sodium ethanoate in ethanoic acid. The final solution contains ethanoic acid at 0.250 mol dm⁻³ and sodium ethanoate at 0.150 mol dm⁻³. For ethanoic acid, = 1.74 × 10⁻⁵ mol dm⁻³ at 298 K.Calculate the pH of the buffer solution.2 marks
- A student prepares 100 cm³ of a buffer solution containing 0.0500 mol of ethanoic acid and 0.0500 mol of sodium ethanoate at 298 K. For ethanoic acid, = 1.74 × 10⁻⁵ mol dm⁻³. She then adds 0.00500 mol of solid sodium hydroxide. Assume that adding the solid does not change the volume.Calculate the pH of the solution after the sodium hydroxide has been added.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).