pH & Buffer Lab
25 °C · Ideal solutions · [H₃O⁺][OH⁻] = Kw = 1.0 × 10⁻¹⁴ · pH + pOH = 14.00
1. Set up
Your solution
Strong acid reference
Strong acids: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄. Not on the list? Treat it as weak.
H₂SO₄ is listed for classification only. It is not used in these pH calculations.
2. Observe
Equilibrium measurements
- pH
- pOH
- [H₃O⁺]
- [OH⁻]
Particle view
Schematic particles, not a count of solvent molecules. Very small ion fractions may be too small to draw.
Student method and exact answer
Decimal places in pH equal significant figures in concentration. Readouts use two decimal places for two significant figures.
1. Set up
Build a buffer
HA and A⁻ label the acid and conjugate base, even when their actual charges differ.
2. Predict and compare
Henderson-Hasselbalch
pH = pKa + log [A⁻][HA]
- pH from method
- Exact pH
Stress Test starts with equal amounts of acid and conjugate base. Set total concentration there.
1. Set up and add
Compare buffer capacity
Each beaker starts at 100.0 mL. The buffer starts with equal moles of HA and A⁻. Changing the pair or capacity starts a new experiment.
Add to both beakers
HCl and NaOH are 0.10 M.
2. Compare
Water and buffer
Pure water
Buffer
pH vs. volume added
Water (dashed) Buffer (solid)
Volume added is cumulative: HCl plus NaOH. Additions in either direction stay on the curve.
Moles in the buffer
Bars show equilibrium moles. Near exhaustion, equilibrium can regenerate a small amount of the depleted component.
Data table
| Addition | Volume added (mL) | HCl total (mL) | NaOH total (mL) | pH water | pH buffer |
|---|
1. Design
Make a buffer at pH X
Pick a pair with pKa within 1 of the target. Set [A⁻]/[HA] to reach the target within 0.05 pH.
2. Maintain
Keep the blood at 7.35 to 7.45
Use H₂CO₃ / HCO₃⁻. Adjust [HCO₃⁻]/[H₂CO₃] to keep the model in the target range.
This is a closed, ideal carbonate model at 25 °C with pKa 6.35. Real blood is an open system regulated by lungs and kidneys, with other buffers and a different temperature.
3. Detect
Pick the indicator
Choose the indicator with pKa closest to the equivalence point. Its transition range is approximately pKa ± 1.
About this activity
Measure the pH of acids and bases, then design a buffer. Add acid or base to compare how well your buffer and plain water resist a change in pH.
Related lessons
- pH CalculationsChemistry
- BuffersChemistry
- 8.8 Properties of BuffersAP Chemistry