Chemistry 11–12 · Year 12

Titrating sodium hydroxide of unknown concentration against a standard acid

Module 6: Acid/Base Reactions

Practical, model not builtLow risk

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The idea

The volume of a standard solution needed to reach the end point, with the mole ratio of the equation, fixes the unknown concentration to four significant figures.

What you need

0.1000 mol/L hydrochloric acid, 150 mL, standardised beforehand against the 0.100 mol/L sodium carbonate primary standard from Module 2 with methyl orange
Sodium hydroxide solution of unknown concentration near 0.1 mol/L, 150 mL
50.00 mL burette, burette clamp and stand, small funnel, white tile
25.00 mL bulb pipette and filler, 250 mL conical flasks, three
Phenolphthalein (for HCl/NaOH) or methyl orange (for Na2CO3/HCl), dropper bottle; wash bottle of deionised water

How to do it

Rinse the burette with the acid and fill it, removing the air bubble from the tip; rinse the pipette with the base.
Pipette 25.00 mL of the base into a flask, add two drops of phenolphthalein (pink).
Run in acid quickly to within 2 mL of the expected end point (rough titre), then dropwise with swirling until the pink just vanishes and stays gone for 30 s; read the burette to 0.05 mL.
Repeat until three titres agree within 0.10 mL; average the concordant titres.
Calculate moles of acid delivered, moles of base in the aliquot (1:1) and the base concentration to four significant figures.
In the simulation enter the same titre and compare; then vary the standard's concentration and the aliquot to see the effect on the result and its uncertainty.

What you should see

A rough titre followed by three concordant titres, for example 24.55, 24.60 and 24.65 mL (mean 24.60 mL), gives a base concentration of 0.0984 mol/L. The end point with phenolphthalein is a sharp pink to colourless change on one drop; overshooting by one drop (0.05 mL) shifts the result by 0.2 percent. Titres that drift downward between repeats can mean the base is absorbing carbon dioxide from the air: each carbon dioxide removes two hydroxide ions, but the carbonate formed takes only one hydrogen ion by the phenolphthalein end point.

What changes

What you change: volume of standard acid delivered
What you measure: indicator colour at the end point, hence the titre (mL) and the calculated concentration
What you keep the same: aliquot volume
standard concentration
indicator and number of drops
same burette and reader

Common misconceptions

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

The end point is where the solution becomes neutral (with phenolphthalein it is pH 8.3 to 10; the equivalence point is pH 7 for this pair, and the two differ by less than a drop).
More indicator makes the end point sharper (indicator is itself a weak acid; two drops is enough and more shifts the result).
Rinsing the flask with base before pipetting is good technique (it adds extra base; the flask is rinsed with water only).

Safety card

Low riskLearners carry it out

Hazards

sodium hydroxide about 0.1 mol/L irritant
hydrochloric acid 0.1 mol/L irritant
burette above eye level

Controls

fill the burette below eye level using a funnel, then remove the funnel
pipette filler
eye protection

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
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 moduleCH11/12-4
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-02 CH-12WS-02 CH-12WS-03 CH-12WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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