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Pearson’s revised context neutralises one measured aliquot of weak acid with sodium hydroxide, then adds a second equal aliquot of the original acid. The first aliquot becomes conjugate base A⁻; the second supplies HA. For equal original aliquots and accurate neutralisation, n(HA) ≈ n(A⁻), giving a half-neutralised mixture. This is a practical alternative to locating half-equivalence on a complete titration curve.
Video by Eliot Rintoul. The video explains buffer ratios. The written practical applies them to Pearson’s two-equal-aliquot preparation and its measurement errors.
Viewing focus: Prepare a half-neutralised weak-acid mixture
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Why add a second equal acid aliquot after neutralising the first?
To produce approximately equal HA and A⁻ amounts. The original acid and its conjugate base then form a half-neutralised buffer.
After watching 2: At the intended equal ratio, what does pH approximately equal?
pKa. Ka = [H⁺][A⁻]/[HA] simplifies when the ratio is approximately one.
What you'll be able to do
Pearson’s revised context neutralises one measured aliquot of weak acid with sodium hydroxide, then adds a second equal aliquot of the original acid. The first aliquot becomes conjugate base A⁻; the second supplies HA. For equal original aliquots and accurate neutralisation, n(HA) ≈ n(A⁻), giving a half-neutralised mixture. This is a practical alternative to locating half-equivalence on a complete titration curve.
Use the same original acid solution and calibrated volumetric pipette for both aliquots. Record the actual method and end-point evidence. Do not add NaOH to the second aliquot as though starting another full titration: that would destroy the intended acid-to-base ratio. Use the approved dilute solutions, indicator and handling arrangements.
Calibrate the pH meter with suitable buffers at the measurement temperature, rinse appropriately and obtain a stable reading in the thoroughly mixed solution. In the weak-acid expression Ka = [H⁺][A⁻]/[HA], an approximately equal concentration ratio makes Ka ≈ [H⁺]. Therefore pKa ≈ pH and Ka ≈ 10⁻ᵖᴴ under the concentration approximation.
The mixture is a buffer containing both acid and conjugate base. The square-root expression [H⁺] ≈ √(Kac), used for an initially pure weak acid, is not the appropriate starting equation here. An indicator changes over a range and does not measure the buffer’s pH accurately enough to replace the calibrated meter.
If the first aliquot is over-neutralised, excess hydroxide consumes some of the second acid aliquot. The final A⁻/HA ratio becomes greater than one and the measured pH can exceed pKa. If the two acid aliquots differ in volume or concentration, they also fail to produce the intended ratio. Distinguish those chemical preparation errors from a probe calibration offset.
Temperature affects both electrode response and acid dissociation, so record it for comparison with a reference Ka. Equal amounts give a useful approximation, while activity effects and dissociation mean the result need not be exact. Repeat independent preparations to assess scatter; do not average readings from one misprepared mixture and claim this removes its ratio error.
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 first aliquot contains 2.00 mmol HA, but 2.20 mmol OH⁻ is added before a second 2.00 mmol HA aliquot. Find the final A⁻:HA ratio.
Ratio 2.20/1.80 = 1.22. pH is above pKa, so treating it as pKa underestimates Ka.
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.