Investigating Science 11–12 · Year 12

Testing a label claim: how much acid does an antacid tablet neutralise?

Module 7: Fact or Fallacy?

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

An efficacy claim on a medicine label can be tested quantitatively: a back-titration measures how much acid one tablet neutralises and compares it with the stated active ingredient.

Safety card

Medium riskA teacher supervises

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

Hazards

1.00 mol/L hydrochloric acid irritates eyes and skin
0.500 mol/L sodium hydroxide can damage eyes
no tasting or swallowing of tablets in the laboratory

Controls

safety glasses throughout
fill the burette below eye level with a funnel removed before titrating
wipe spills with plenty of water

Note

Chemicals are used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1 (general information for all staff, including the CSIS 1.7 risk assessment requirement), read the Safety Data Sheet for each chemical and complete a RiskAssess risk assessment before the lesson.

What you need

antacid tablets whose only active ingredient is calcium carbonate, with its mass per tablet on the label (for example 500 mg)
1.00 mol/L hydrochloric acid and a 20.0 mL pipette; 0.500 mol/L sodium hydroxide in a 50 mL burette
methyl orange indicator, conical flasks, mortar and pestle, white tile

How to do it

Titrate 20.0 mL of the acid alone with the sodium hydroxide as a blank.
Crush one tablet and add it to 20.0 mL of the acid; swirl until fizzing stops (about 5 minutes).
Add methyl orange and titrate the excess acid with sodium hydroxide to the orange end point.
Repeat with three tablets and calculate the amount of acid each neutralised.
Compare with the amount the label’s calcium carbonate content predicts, and evaluate the validity, reliability and accuracy of the test.

What you should see

A tablet containing 500 mg of calcium carbonate (4.995 mmol) neutralises 9.99 mmol of hydrochloric acid (CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂). From 20.0 mmol of acid, 10.0 mmol remain, needing 20.0 mL of 0.500 mol/L sodium hydroxide; the blank needs 40.0 mL. A titre of 20.0 ± 0.3 mL supports the label to within 1.5 %; a tablet with only 90 % of the stated carbonate would need 22.0 mL. Methyl orange changes colour before dissolved carbon dioxide reacts, so the carbon dioxide does not disturb the end point.

What changes

What you change: tablet (three replicates)
What you measure: volume of sodium hydroxide needed
What you keep the same: acid volume and concentration
reaction time before titrating
same indicator and end-point colour

Common misconceptions

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

A bigger tablet always neutralises more acid.
Fizzing shows the tablet is working harder.
A claim on a label must be accurate because it is printed.

Curriculum references

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

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS12-14 INS11/12-4 INS11/12-5 INS11/12-6 INS11/12-7
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities