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Chemistry 11–12 · Year 12

Measuring the pH of strong and weak acids and bases across three concentrations with a calibrated pH probe

Module 6: Acid/Base Reactions

Practical, model not builtLow risk

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The idea

pH depends on both the concentration and the degree of dissociation: a strong acid's pH falls by one unit per tenfold concentration while a weak acid's falls by only half a unit.

What you need

  • pH meter or probe with logger, calibrated with pH 4.00 and pH 7.00 buffers (and pH 10.00 for the bases)
  • 0.100 mol/L hydrochloric acid, 0.100 mol/L ethanoic acid, 0.100 mol/L sodium hydroxide, 0.100 mol/L ammonia, 100 mL each
  • Volumetric pipettes 10.00 mL and 100.0 mL volumetric flasks for two serial tenfold dilutions of each
  • Deionised water (freshly boiled and cooled for the base dilutions), 50 mL beakers, wash bottle, tissues

How to do it

  1. Calibrate the probe with the two buffers; rinse with deionised water between every solution and blot, never wipe.
  2. Dilute each 0.100 mol/L stock tenfold and then a further tenfold, to give 0.0100 and 0.00100 mol/L.
  3. Measure the pH of the twelve solutions, allowing the reading to settle for 30 s each; measure deionised water last.
  4. Tabulate pH against concentration for each substance; plot pH against log10(concentration) and find the gradient for each line.
  5. Calculate the expected pH of each hydrochloric acid and sodium hydroxide solution and, using the simulation, of each ethanoic acid and ammonia solution; compare with the readings.
  6. From the measured pH of 0.100 mol/L ethanoic acid calculate Ka and compare with the accepted value.

What you should see

Hydrochloric acid reads close to 1.0, 2.0 and 3.0 and sodium hydroxide close to 13.0, 12.0 and 11.0 (gradient of pH against log concentration is -1 and +1). Ethanoic acid reads about 2.88, 3.39 and 3.91 (gradient near -0.5; degree of dissociation rising from 1.3 percent to 12 percent on dilution) and ammonia about 11.1, 10.6 and 10.1. Deionised water reads below 7 because it absorbs carbon dioxide from the air. Readings well away from these values point to a probe that needs recalibrating, or to dilute bases that have absorbed carbon dioxide.

What changes

What you change
concentration and identity of the acid or base
What you measure
pH
What you keep the same
  • temperature
  • probe calibration
  • settling time
  • fresh deionised water for dilutions

Common misconceptions

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

  • A pH of 3 means the acid is weak (0.001 mol/L hydrochloric acid also has pH 3; strength and concentration are different things).
  • Diluting a weak acid makes it stronger (it raises the degree of dissociation; the acid is still weak, Ka unchanged).
  • pH 7 is always neutral (only at 25 degrees Celsius; Kw rises with temperature).

Safety card

Low riskLearners carry it out

Hazards

  • 0.1 mol/L sodium hydroxide corrosive
  • ammonia solution irritant vapour
  • glass probe fragile

Controls

  • eye protection
  • ammonia stock kept stoppered
  • probe stored in its storage solution

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)CH12-13CH11/12-3
  • 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 6, which it allows in any moduleCH11/12-4CH11/12-6
  • Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-02CH-12WS-03CH-12WS-06
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/chemistry-stage-6-2017
  2. www.nsw.gov.au/sites/default/files/noindex/2025-03/chemistry-stage6-syllabus-word.docx
  3. curriculum.nsw.edu.au/learning-areas/science/chemistry-11-12-2025/outcomes
  4. edu.rsc.org/experiments/testing-the-ph-of-different-solutions/395.article
  5. edu.rsc.org/cpd/teaching-acids-and-bases-post-16/4013534.article
  6. education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/chemistry/Chemistry_module_6_IQ3.docx

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