Chemistry 11–12 · Year 12

Analysing household substances: the ethanoic acid content of vinegar and a back titration of an antacid tablet

Module 6: Acid/Base Reactions

PracticalLow risk

The idea

Titration measures the acid or base content of everyday products, with a back titration used when the sample is a solid that reacts slowly or is insoluble.

What you need

White vinegar, 25 mL, diluted tenfold in a 250.0 mL volumetric flask
0.100 mol/L sodium hydroxide (standardised), 150 mL; phenolphthalein
Antacid tablet (calcium carbonate based, 500 mg stated), one, crushed and weighed
0.500 mol/L hydrochloric acid (standardised), 60 mL; methyl orange
Burette, 50.00 mL and 25.00 mL pipettes, 250.0 mL volumetric flask, conical flasks, 250 mL beaker, hotplate, balance to 0.01 g

How to do it

Vinegar: pipette 25.00 mL of the diluted vinegar into a flask, add phenolphthalein and titrate with sodium hydroxide to the first permanent pink; repeat to three concordant titres.
Calculate the ethanoic acid concentration of the diluted and the original vinegar in mol/L and in grams per 100 mL; compare with the label.
Antacid: weigh the crushed tablet into a beaker, add exactly 50.00 mL of 0.500 mol/L hydrochloric acid by pipette, warm gently for five minutes to drive off carbon dioxide, cool, and transfer quantitatively to the 250.0 mL volumetric flask; make up to the mark.
Titrate 25.00 mL aliquots of this solution with 0.100 mol/L sodium hydroxide using methyl orange to the orange end point until three titres agree within 0.10 mL.
Calculate the acid neutralised by the tablet, the mass of calcium carbonate it contains and the percentage by mass; compare with the label.
Optional: repeat the vinegar method for lemon juice or a soft drink (degassed by stirring) using a pH probe to find the end point.

What you should see

A vinegar labelled 4.0 g of acetic acid per 100 mL is 0.666 mol/L; after tenfold dilution a 25.00 mL aliquot needs 16.7 mL of 0.100 mol/L sodium hydroxide, and the learner's result should agree with the label within the titration's uncertainty plus the label's rounding. A tablet holding 500 mg of calcium carbonate (5.00 mmol) consumes 9.99 mmol of the 25.00 mmol of acid added, leaving 15.01 mmol; after making up to 250.0 mL each 25.00 mL aliquot holds 1.501 mmol and needs 15.01 mL of 0.100 mol/L sodium hydroxide. Methyl orange changes colour in the acid range, where dissolved carbon dioxide stays almost entirely as carbonic acid, so any carbon dioxide left in the solution takes almost no sodium hydroxide and barely changes the titre; with phenolphthalein it would use extra sodium hydroxide and make the calculated calcium carbonate low. The warming makes sure the tablet has fully reacted and stopped fizzing before the transfer; a result above the label can mean the tablet contains another base as well.

What changes

What you change: the product analysed (brand or type)
What you measure: acid content (mol/L and g/100 mL) or base content (mg calcium carbonate per tablet)
What you keep the same: standard concentrations
aliquot volume
indicator
complete removal of carbon dioxide before the back titration

Common misconceptions

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

The label percentage is by volume of acid (it is grams of acetic acid per 100 mL; the titration gives moles, which convert through the molar mass).
A back titration measures the tablet directly (it measures the acid left over; the tablet's base is found by difference).
Warming the mixture loses acid (warming dilute hydrochloric acid gently for five minutes drives off carbon dioxide, not hydrogen chloride).

Safety card

Low riskLearners carry it out

Hazards

0.1 mol/L sodium hydroxide irritant
hot beaker
carbon dioxide frothing on adding acid to the tablet

Controls

add acid slowly to the crushed tablet
eye protection
hotplate rather than flame

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.

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.

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