Chemistry 11–12 · Year 12

One molar hydrochloric acid against one molar ethanoic acid: conductivity, pH and rate with magnesium and marble

Module 6: Acid/Base Reactions

PracticalMedium risk

School laboratory, not for home

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

The idea

Two acids of the same concentration differ in hydrogen ion concentration if one is only partly dissociated, and the difference shows in conductivity, pH and reaction rate but not in the amount of base needed to neutralise them.

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 acids irritant; ethanoic acid vapour stings
1.0 mol/L sodium hydroxide corrosive
hydrogen flammable

Controls

eye protection
no flames
burette filled below eye level with a funnel
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

1.0 mol/L hydrochloric acid and 1.0 mol/L ethanoic acid, 100 mL each (the magnesium, marble and three titration portions take 60 mL of each; the rest covers the pH and conductivity readings)
Conductivity probe, pH probe, both calibrated
Magnesium ribbon 2 cm pieces, four, weighed; marble chips 1 g, four
Test tubes, gas syringe or inverted measuring cylinder, stopwatch
1.0 mol/L sodium hydroxide 100 mL, burette, phenolphthalein, 10.00 mL pipette

How to do it

Measure the conductivity and the pH of each acid at 25 degrees Celsius.
Add a weighed 2 cm piece of magnesium to 10 mL of each acid and time the disappearance of the metal; repeat once.
Add 1 g of marble chips to 10 mL of each acid and record the gas volume after 30 s and after two minutes.
Titrate three 10.00 mL portions of each acid with 1.0 mol/L sodium hydroxide using phenolphthalein; record the titres.
Tabulate all five measures and explain which depend on hydrogen ion concentration and which on the total acid present.
Predict the hydrogen ion concentration of the ethanoic acid from its pH and compare with 1.0 mol/L.

What you should see

Hydrochloric acid reads pH 0.0 to 0.2 and a conductivity many times that of ethanoic acid, which reads pH 2.3 to 2.5 (hydrogen ion concentration about 4 mmol/L, roughly 0.4 percent dissociated). Magnesium fizzes hard and is gone within about a minute in hydrochloric acid but takes several minutes in ethanoic acid; marble gives gas much faster in hydrochloric acid. Both acids need the same titre, 10.0 mL of 1.0 mol/L sodium hydroxide, because the total acid is the same.

What changes

What you change: identity of the acid at equal concentration
What you measure: conductivity, pH, time for magnesium to react, gas volume from marble, titre
What you keep the same: concentration 1.0 mol/L
temperature
magnesium mass and marble mass
volumes

Common misconceptions

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

Ethanoic acid is weak because it is dilute (it is 1.0 mol/L here; weak means partly dissociated).
A weak acid needs less base to neutralise it (it needs the same; the base pulls the equilibrium to completion).
The slower reaction means ethanoic acid contains fewer acid molecules (it contains as many; fewer are dissociated at any instant).

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 CH11/12-1 CH11/12-3 CH11/12-5
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-02 CH-12WS-01 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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