Chemistry 11–12 · Year 12

Gravimetric analysis of the sulfate content of a lawn fertiliser or water sample as barium sulfate

Module 8: Applying Chemical Ideas

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

Precipitating an ion completely as an insoluble compound of known formula, then drying and weighing it, gives the ion's mass in the original sample.

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

barium chloride is toxic if swallowed
hot beaker and hotplate
1 mol/L hydrochloric acid irritant
silver nitrate is corrosive and stains skin

Controls

gloves for barium chloride and silver nitrate
barium wastes collected
tongs for hot glassware
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.

What you need

Soluble lawn fertiliser (sulfate of ammonia type) 0.500 g, or 25.00 mL of a 0.100 mol/L sodium sulfate solution as the check sample
0.20 mol/L barium chloride, 50 mL; 1 mol/L hydrochloric acid, 5 mL; 0.1 mol/L silver nitrate in a dropper bottle, 2 mL, for testing the washings
250 mL beaker, glass rod, hotplate, watch glass, wash bottle
Ashless filter paper or a pre-weighed sintered glass crucible, funnel, drying oven at 105 degrees Celsius, desiccator, balance to 0.001 g

How to do it

Dissolve the weighed sample in 100 mL deionised water, add 2 mL of hydrochloric acid, and heat to near boiling.
Add 30 mL of the barium chloride (6.0 mmol, a clear excess over the 2.5 to 3.8 mmol of sulfate) slowly with stirring; keep hot for 30 minutes so the fine precipitate coarsens.
Let it settle; test the clear liquid with a further drop of barium chloride to confirm precipitation is complete.
Filter through the weighed paper or crucible; wash the precipitate three times with hot water until a drop of the washings gives no cloudiness with silver nitrate.
Dry at 105 degrees Celsius for an hour, cool in the desiccator and weigh; repeat drying until constant mass.
Calculate moles of barium sulfate, hence moles and mass of sulfate in the sample and the percentage by mass; compare with the label or the known solution; use the simulation to see how an incomplete wash or damp precipitate biases the result.

What you should see

The check sample of 25.00 mL of 0.100 mol/L sodium sulfate (2.500 mmol) gives 0.5835 g of barium sulfate ideally; high results usually mean chloride left in the precipitate or incomplete drying, low results mean fines passed the paper. Pure ammonium sulfate is 24.3 percent sulfur, so 0.500 g (3.78 mmol of sulfate, against 6.0 mmol of barium added) gives 0.883 g of barium sulfate; a fertiliser's result is compared with the sulfur percentage on its label. The learner knows the precipitation was complete when a drop of barium chloride added to the clear supernatant gives no further cloudiness.

What changes

What you change: sample analysed
What you measure: mass of barium sulfate precipitate (g) and the sulfate percentage of the sample
What you keep the same: excess barium chloride
digestion time and temperature
washing to a negative chloride test
drying to constant mass

Common misconceptions

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

A bigger precipitate means a purer sample (excess mass usually means impurities or water were weighed too).
Washing the precipitate dissolves it away (from its Ksp, 50 mL of wash water at 25 degrees Celsius dissolves about 0.12 mg, about 0.02 percent of a 0.58 g precipitate).
Any reagent will do for gravimetric analysis (the precipitate must be insoluble, pure, of known formula and stable on drying).

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)CH12-15 CH11/12-2 CH11/12-3 CH11/12-4
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 8, which it allows in any moduleCH11/12-6
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-04 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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