Gravimetric analysis is a quantitative chemistry method that determines how much of a substance is present by converting it into a solid of known composition. It matters because mass can be measured very precisely with an analytical balance, making this method highly accurate when done carefully. A common approach is to add a reagent that forms an insoluble precipitate with the ion being studied, then collect, dry, and weigh that precipitate.
The measured mass is used with stoichiometry to calculate the amount or percent of the original analyte.
Key Facts
- Precipitation reaction: analyte ion + reagent ion -> insoluble precipitate
- Moles of precipitate = mass of precipitate / molar mass of precipitate
- Stoichiometry links moles of precipitate to moles of analyte using the balanced equation.
- Mass of analyte = moles of analyte x molar mass of analyte
- Percent by mass = mass of analyte / mass of sample x 100%
- A good gravimetric precipitate is insoluble, pure, stable when dried, and easy to filter.
Vocabulary
- Gravimetric analysis
- A quantitative method that determines the amount of an analyte by measuring the mass of a related solid product.
- Analyte
- The chemical substance in a sample that is being measured or identified.
- Precipitate
- An insoluble solid that forms when ions in solution react to make a compound with very low solubility.
- Filtrate
- The liquid that passes through the filter after the precipitate has been collected.
- Constant mass
- A condition reached when repeated drying and weighing give nearly the same mass, showing that water or solvent has been removed.
Common Mistakes to Avoid
- Skipping the balanced equation, which is wrong because the mole ratio between analyte and precipitate is needed for the calculation.
- Weighing the precipitate before it is fully dry, which is wrong because leftover water increases the measured mass and makes the analyte amount too high.
- Losing solid during transfer or filtration, which is wrong because missing precipitate lowers the measured mass and gives a falsely low result.
- Assuming every cloudy solid is pure product, which is wrong because impurities and unreacted reagent can be trapped in the precipitate and distort the final mass.
Practice Questions
- 1 A 0.500 g sample containing chloride ions is treated with excess AgNO3 to form 0.287 g of AgCl. Using AgCl molar mass = 143.32 g/mol and Cl molar mass = 35.45 g/mol, calculate the percent by mass of chloride in the sample.
- 2 A sulfate sample is precipitated as BaSO4. If 0.466 g of BaSO4 is collected and dried, how many moles of sulfate ions were in the original sample? Use BaSO4 molar mass = 233.39 g/mol and the ratio 1 mol SO4 2- to 1 mol BaSO4.
- 3 Explain why a gravimetric analysis can give a result that is too high if the precipitate is not washed properly, even if none of the solid is lost.