Chemistry 11–12 · Year 11

Magnesium and hydrochloric acid in an inverted burette: whole-number mole ratio and the molar volume of a gas

Module 2: Introduction to Quantitative Chemistry

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

One mole of magnesium releases one mole of hydrogen, and the volume of that gas at the room's temperature and pressure gives the molar volume.

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

2 mol/L hydrochloric acid is irritant
hydrogen is flammable
acid can run out of the burette while inverting

Controls

invert over the beaker with a finger firmly over the opening
no flames near the bench
eye protection
rinse spills with water

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

Magnesium ribbon, 3.5 cm lengths cleaned with emery paper, each weighed to 0.001 g (0.020 to 0.040 g)
2.0 mol/L hydrochloric acid, 25 mL per trial
50 mL burette, burette stand and clamp
250 mL beaker, small funnel
Deionised water, 100 mL
Thermometer, barometer or a weather-service pressure reading for the room

How to do it

Pour 25 mL of acid into the burette through the funnel, then trickle 25 mL of water down the inside wall so it floats on the acid without mixing.
Bend the magnesium and wedge it in the open top of the burette by its own spring, 2 cm inside; add 50 mL water to the beaker.
Cover the top with a finger, invert the burette quickly into the beaker and clamp it vertically; record the initial reading.
Wait while the acid diffuses down and reacts with the magnesium (5 to 10 minutes); when no metal remains and bubbles stop, record the final reading.
Record room temperature and pressure; compute moles of magnesium, the gas volume, the volume per mole and the ratio n(H2):n(Mg).
Repeat with two more weighed strips; use the simulation to see how the molar volume the class should obtain shifts with the day's temperature and pressure.

What you should see

Every mole of magnesium gives one mole of hydrogen (Mg + 2 HCl gives MgCl2 + H2). A 0.035 g strip releases 1.44 mmol of hydrogen: about 35.7 mL at 25 degrees Celsius and 100 kPa, or 34.9 mL at 22 degrees Celsius and 101.3 kPa. The class molar volume should approach the computed 24.79 L/mol at 25 degrees Celsius and 100 kPa. The collected gas is saturated with water vapour (about 3.17 kPa at 25 degrees Celsius), so an uncorrected result reads about 3.3 percent high; subtracting the vapour pressure from the room pressure removes that bias.

What changes

What you change: mass of magnesium
What you measure: volume of hydrogen collected (mL)
What you keep the same: acid concentration and volume
temperature during collection
time allowed for complete reaction

Common misconceptions

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

One mole of any gas is 22.4 L (that figure holds only at 0 degrees Celsius and 101.325 kPa; at 25 degrees Celsius and 100 kPa it is 24.8 L).
More acid makes more hydrogen (the magnesium is limiting; the acid is in large excess).
The gas volume measures the mass of hydrogen directly (it measures moles; mass needs the molar mass 2.016 g/mol).

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)CH11-9 CH11/12-4 CH11/12-6
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 2, which it allows in any moduleCH11/12-3
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-11-02 CH-11WS-03 CH-11WS-06
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities