HSC · Topic explainer
Using a precipitate to measure sulfate by mass
Gravimetric analysis uses a measured mass to determine the amount of an analyte. For sulfate, a known precipitation reaction can convert dissolved sulfate into barium sulfate. Determine the moles of the dry precipitate, use the balanced mole ratio, then express sulfate mass as a percentage of the original sample.
Worked example
A 2.00 g sample produces 0.233 g of dry BaSO₄. Using a molar mass of about 233 g/mol gives 0.00100 mol of precipitate. The 1:1 mole ratio gives the same amount of sulfate.
Multiply 0.00100 mol by 96.1 g/mol to obtain about 0.0961 g sulfate, or 4.81% by mass in the sample. An undried solid can make the result too high, while incomplete precipitation can make it too low.
Official exam practice
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