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An acidic buffer contains a weak acid and its conjugate base, often supplied as a soluble salt. Added H+ reacts mainly with the conjugate base to form HA; added OH⁻ reacts mainly with HA to form water and A⁻. The large reservoirs prevent a large ratio change for small additions, so pH changes only a little. A buffer does not keep pH perfectly constant and can be overwhelmed when one component is nearly consumed.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain how acidic buffers resist pH change
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What is pH at half-neutralisation of a weak acid by a strong base?
pKa. The weak acid and conjugate base amounts are equal.
After watching 2: Which component mainly removes added H+ in an acidic buffer?
The conjugate base. A⁻ accepts the proton to form HA.
What you'll be able to do
An acidic buffer contains a weak acid and its conjugate base, often supplied as a soluble salt. Added H+ reacts mainly with the conjugate base to form HA; added OH⁻ reacts mainly with HA to form water and A⁻. The large reservoirs prevent a large ratio change for small additions, so pH changes only a little. A buffer does not keep pH perfectly constant and can be overwhelmed when one component is nearly consumed.
Rearrange Ka to . If both species share the same volume, their amount ratio can replace their concentration ratio. When strong acid or alkali is added, first calculate how many moles react and update both buffer components. Only then apply the equilibrium expression. At half-neutralisation of a weak acid by a strong base, amounts of HA and A⁻ are equal and pH equals pKa. Simple dilution approximately preserves the ratio but reduces buffer capacity.
Equivalence means stoichiometrically equal reacting amounts, not necessarily pH 7. A weak acid–strong base equivalence point is alkaline because the conjugate base reacts with water; the opposite pairing is acidic. Choose an indicator whose transition range lies within the steep section of the pH curve. A weak acid–weak base titration generally has too little steep change for a sharp ordinary indicator endpoint. Measure more densely near equivalence and use the actual volume of added titrant when converting amounts into concentrations.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: The same buffer receives 0.0200 mol HCl. Find the new pH, assuming the buffer remains effective.
pH 5.18.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.