Chemistry 11–12 · Year 12

Preparing an ethanoic acid and ethanoate buffer and testing how it resists added acid and base

Module 6: Acid/Base Reactions

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

A mixture of a weak acid and its conjugate base holds its pH when small amounts of acid or base are added, because the added ions are absorbed by the equilibrium between the two forms.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

1.0 mol/L sodium hydroxide corrosive
1.0 mol/L hydrochloric acid irritant
ethanoic acid vapour

Controls

1 mL droppers or syringes for the strong reagents
eye protection
spills rinsed

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions.

What you need

0.20 mol/L ethanoic acid, 150 mL, and 0.20 mol/L sodium ethanoate, 150 mL: three 100 mL batches of buffer, one for each run (or, for each batch, 100 mL of 0.20 mol/L ethanoic acid half-neutralised with 10.0 mL of 1.0 mol/L sodium hydroxide, which gives the same 10 mmol of each form, 0.091 mol/L each in 110 mL)
1.0 mol/L hydrochloric acid and 1.0 mol/L sodium hydroxide in 1 mL graduated droppers or a 2 mL syringe
Calibrated pH probe, 100 mL beakers, five (three buffer and two water), deionised water, 250 mL, stirrer

How to do it

Make 100 mL of buffer by mixing 50.0 mL of the acid with 50.0 mL of the ethanoate (each then 0.10 mol/L); measure its pH.
Put 100 mL of deionised water in a second beaker and measure its pH.
Add 1.0 mL of 1.0 mol/L hydrochloric acid to each beaker, stir, and record the pH of each.
Repeat with fresh buffer and fresh water, adding 1.0 mL of 1.0 mol/L sodium hydroxide.
Keep adding acid to a fresh buffer 1.0 mL at a time, recording the pH each time, until the pH has fallen by more than one unit; record the volume at which the buffer failed.
Compare each reading with the Henderson-Hasselbalch prediction in the simulation and explain the capacity limit.

What you should see

The buffer reads pH 4.76 (equal to pKa, since the two forms are equal). Adding 1.0 mL of 1.0 mol/L acid moves it only to 4.67 and 1.0 mL of base to 4.84; the same acid added to 100 mL of water takes it to pH 2.00 and the same base to pH 12.00. The buffer stays within one pH unit of pKa until 8.2 mL of the 1.0 mol/L acid has been added (ethanoate to ethanoic acid ratio 1:10), then falls steeply as the last of its 10 mmol of ethanoate is used up.

What changes

What you change: volume of strong acid or base added, and whether the solution is buffer or water
What you measure: pH after each addition
What you keep the same: buffer concentrations and volume
temperature
probe calibration
stirring

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

A buffer keeps the pH at exactly 7 (it holds the pH near its own pKa, 4.76 here).
A buffer can absorb any amount of acid (its capacity is limited by the moles of the conjugate base present).
A buffer stops the added acid from reacting (the acid reacts fully with the ethanoate; that is why the pH holds).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-13 CH11/12-2 CH11/12-3 CH11/12-5
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-02 CH-12WS-02 CH-12WS-03 CH-12WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities