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Buffer Solutions Study

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Buffer Solutions Study

A local water-testing team needs to keep samples within a target pH range. Use the data and chemistry in this worksheet to explain buffer action and make justified calculations. For logarithmic calculations, give pH to 2 decimal places unless stated otherwise. Assume concentrations in the Henderson–Hasselbalch equation refer to equilibrium concentrations; where a small addition is considered, use stoichiometric changes first.

Buffer action and equilibria

The water-testing team prepares an acidic buffer from ethanoic acid, CH₃COOH, and sodium ethanoate, CH₃COONa. The relevant equilibrium is CH₃COOH ⇌ H⁺ + CH₃COO⁻.

1.Which description best explains what a buffer does when small amounts of acid or alkali are added?

It keeps the pH exactly constant under all conditions.
It reduces the change in pH by reacting with added H⁺ or OH⁻.
It neutralises any quantity of acid or alkali completely.
It maintains a solution at pH 7.

2.Explain, using the equilibrium species, how this buffer responds when a small amount of hydrochloric acid is added. Include an equation and state the effect on the concentrations of CH₃COOH and CH₃COO⁻.

3.A student says, “Any mixture containing a weak acid is a buffer.” Correct this statement by giving the essential components of an acidic buffer and explaining the role of each component.

4.Which of these mixtures can act as buffers? Select every suitable answer.

CH₃COOH and CH₃COONa
HCl and NaCl
NH₃ and NH₄Cl
NaOH and NaCl

Buffer calculations

For an acidic buffer, use pH = pKₐ + log₁₀([A⁻]/[HA]). Ethanoic acid has pKₐ = 4.76. Assume the added volumes are negligible compared with the buffer volume.

5.A 1.00 L buffer contains 0.100 mol CH₃COOH and 0.150 mol CH₃COO⁻. Calculate its initial pH.

6.To the buffer in the previous question, 0.010 mol HCl is added. Calculate the new pH. First show the reaction of H⁺ with the buffer and update the two mole amounts.

7.An ammonia buffer has [NH₃] = 0.200 mol dm⁻³ and [NH₄⁺] = 0.100 mol dm⁻³. Its pH is 9.25. Using pH = pKₐ + log₁₀([NH₃]/[NH₄⁺]), calculate pKₐ for NH₄⁺.

8.A technician can prepare one ethanoate buffer with equal concentrations of CH₃COOH and CH₃COO⁻, or another with ten times as much CH₃COO⁻ as CH₃COOH. Predict the pH of each using pKₐ = 4.76, then identify which mixture has the higher pH.

Choosing and testing a buffer

A buffer has a finite capacity: if too much acid or alkali is added, one buffer component is used up and the pH can change sharply. Select a sensible experiment and interpret results as a chemist would.

9.Match each buffer system to its most relevant setting.

Bicarbonate / carbonic acid
Phosphate buffer
Ethanoic acid / ethanoate

Intracellular pH control
Acidity control in some foods
Blood pH regulation

10.A practical compares equal volumes of distilled water and an ethanoic acid/ethanoate buffer. Describe a fair method to compare their resistance to pH change when acid is added. Include one control variable and the measurements to record.

11.Blood is normally maintained near pH 7.35–7.45. Explain why a large pH change is dangerous, and why a buffer cannot protect blood against an unlimited acid load.

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