Gravimetric analysis – precipitation, filtration and drying to constant mass
ChemistryApplied & Practical ChemistryAges 17–18
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Sign in to playA virtual gravimetric analysis lab. Precipitate sulfate as BaSO₄, chloride as AgCl or the calcium in hard water as CaCO₃, test the clear liquid until precipitation is complete, filter, wash, then heat, cool and weigh until two readings agree; or heat a sodium hydrogencarbonate mixture or hydrated barium chloride to constant mass. Work out the mass percentage of an ion, the water hardness in mg CaCO₃ per dm³ or the formula of a hydrate, and compare the variable composition of a mixture with the fixed composition of a pure substance.
Lesson: Gravimetric analysis: precipitation, filtration, heating to constant mass and percentage composition
What it shows
Gravimetric analysis finds how much of a substance a sample contains by weighing. The ion is precipitated as an insoluble compound of known formula, such as BaSO₄ or AgCl, with the reagent in excess; the precipitate is filtered, washed free of soluble salts and dried to constant mass, and its mass converts to moles and then to the mass of the ion. Heating to constant mass works the same way for solids that lose water or carbon dioxide. A pure substance always gives the same percentage composition, while the composition of a mixture varies from sample to sample.
How to use
Choose an Experiment and a sample, then press Weigh and dissolve. Add the reagent 10 cm³ at a time and press Test clear liquid until no new cloudiness appears. Press Filter and Wash, then Heat, cool, weigh until the last two readings agree, and Record result. Compare your result with the true value, then try skipping a step or using a less precise balance.
Parameters you can change
- Experiment Sulfate in a fertilizer, precipitated as BaSO₄, Chloride in a salt mixture, precipitated as AgCl, Calcium in hard water, precipitated as CaCO₃, Mass % of NaHCO₃ in a mixture by heating, Water of crystallization in hydrated barium chloride
- Sample Unknown mixture (random composition), Pure substance / standard solution
- Mass of solid sample 0.2–2 g
- Volume of water sample 100–1000 cm³
- Balance resolution 0.01 g (2 decimal places), 0.001 g (3 decimal places), 0.0001 g (analytical balance)
Questions to explore
- Why must the reagent be in excess, and how does testing the clear liquid show that it is?
- Why is the precipitate heated, cooled and weighed again and again until two readings agree?
- Why does the pure substance always give the same percentage while the unknown mixture does not?
