Buffer Virtual Lab: Buffer pH and Buffer Capacity

This buffer virtual lab follows the AP Chemistry labs on preparing and testing a buffer. Students add drops of strong acid and strong base to two beakers side by side: distilled water and a buffer. Then they change one thing at a time. The ratio of base form to acid form changes the buffer's starting pH, and the total concentration changes how much acid the buffer can absorb before it fails. Each drop updates both pH meters, a graph of pH against drops, and bars for the two buffer components. Students leave with the two design rules for an effective buffer and the data to back them up.

Curriculum links

• AP Chemistry Unit 8, topics 8.8 Properties of Buffers (SAP-10.B), 8.9 Henderson–Hasselbalch Equation (SAP-10.C) and 8.10 Buffer Capacity (SAP-10.D), and the AP Chemistry labs on preparing an effective buffer and on buffering activity.
• It also fits the buffer topics of IB Chemistry and A-level Chemistry.

Simulic is not affiliated with or endorsed by the College Board or AQA.

Before the lab (5 min)

Give students one fact and ask for a prediction:

"10 drops of 2 M HCl take 50 mL of distilled water from pH 7.00 to pH 1.70. The same 10 drops go into 50 mL of an ethanoate (acetate) buffer at pH 4.76. How far will the buffer's pH fall?"

Offer four choices: less than 0.5 units, about 2 units, about 5 units, or no change at all.

Method in the simulation

Each drop is 0.05 mL of 2 M solution (0.1 mmol). The +10 drops buttons add drops one after another; wait until all of them have landed before reading.

Part 1: water vs buffer

1. Choose the CH₃COOH / CH₃COO⁻ buffer. Set Total concentration to 0.10 M and the ratio to 1.0.
2. Press +10 drops HCl. Record the pH and ΔpH of both beakers.
3. Press Reset, then +10 drops NaOH. Record again.

Part 2: what sets the pH?

4. Keep 0.10 M. Set the ratio to 0.1, then 1.0, then 10. Record the starting pH each time, then add 10 drops of HCl and record ΔpH.

Part 3: what sets the capacity?

5. Set the ratio to 1.0. For total concentrations of 0.05, 0.10 and 0.20 M, add HCl in three steps of 10 drops and record the buffer's pH after each step. Note when the simulation says the buffer capacity is used up.

Total conc. (M)	pH after 10 drops	pH after 20 drops	pH after 30 drops
0.05
0.10
0.20

Expected results

All values are from the simulation (acetate buffer, 50 mL per beaker).

Part 1. Ten drops of HCl take the water from 7.00 to 1.70 (ΔpH −5.30) and the buffer from 4.76 to 4.39 (ΔpH −0.37). Ten drops of NaOH take the water to 12.30 and the buffer to 5.13. The buffer's change is about 14 times smaller.

Part 2.

Ratio [CH₃COO⁻]/[CH₃COOH]	0.1	1.0	10
Starting pH	3.77	4.76	5.76
ΔpH after 10 drops HCl	−1.81	−0.37	−0.61

The starting pH follows pH = pKa + log(ratio). At a ratio of 0.1 there is only 0.45 mmol of base form, so 1.0 mmol of acid uses it up and the pH falls to 1.96. The buffer resists best at a ratio near 1.

Part 3.

Total conc. (M)	10 drops	20 drops	30 drops
0.05	3.82	1.83	1.47
0.10	4.39	3.81	2.01
0.20	4.58	4.39	4.16

All three buffers start at pH 4.76. The simulation's capacity line says the 0.05, 0.10 and 0.20 M buffers can take about 12, 25 and 50 drops of acid. Doubling the concentration doubles the capacity and roughly halves ΔpH for the same addition.

Questions for students

1. (Prediction, asked again after the lab) How far does the buffer's pH fall after 10 drops of HCl?
2. In Part 3, which variables must stay the same?
3. In the 0.10 M buffer with ratio 1.0, what is the pH after 10 drops of HCl?
4. In the 0.20 M buffer, how big is the pH change after 10 drops of HCl?
5. Which conclusion fits your data?

Answers for teachers: (1) Less than 0.5 units (it falls by 0.37). (2) The buffer pair, the ratio and the number of drops. (3) 4.39 (accept 4.37–4.41). (4) 0.18 (accept 0.16–0.20, with or without the minus sign), half the 0.37 change at 0.10 M. (5) The ratio sets the pH; the total concentration sets the capacity.

Common misconceptions

• "A buffer keeps the pH constant." It only slows the change. Every drop moves the pH a little, and past the capacity it falls fast.
• "A more concentrated buffer has a lower pH." At a fixed ratio all three concentrations start at 4.76. Concentration changes capacity, not pH.
• "Any ratio works as long as the pair is right." At a ratio of 0.1 the buffer fails after fewer than 5 drops of acid.

Extension

• Design a buffer: ask for a buffer at pH 7.4 that absorbs at least 30 drops of HCl. Students choose the phosphate pair (pKa 7.21), set a ratio near 1.5 and raise the total concentration until the capacity line allows 30 drops.
• Blood: choose the blood buffer (HCO₃⁻/H₂CO₃ = 20). Students compare how many drops of acid and of base it absorbs, and explain why the body needs the lungs and kidneys too.

FAQ

Do the pH values use the Henderson–Hasselbalch equation?

The simulation shows both. The displayed pH comes from an exact charge balance, so it stays correct after the buffer is used up, when Henderson–Hasselbalch no longer applies.

Why is the water's pH exactly 7.00?

The model assumes the water absorbs no CO₂ from the air. Real distilled water is often near pH 5.6, which makes a good discussion point.

Can students test household products?

Not in this simulation: it has four buffer pairs and pure water. Run the household survey as a wet lab and use this simulation to explain the results.

Related simulations and guides

Acid–Base Titration and the Titration Curve pH scale – measuring and mixing acids and bases

The buffer region also appears in the weak-acid titration curve: see the titration virtual lab. For pH basics, see the pH scale lesson plan, and for more topics, interactive chemistry lesson ideas.