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.
Understanding Chemistry: Gravimetric Analysis
The result depends on more than getting a solid to appear. Chemists choose a reaction in which nearly all of the target ion leaves the solution. If some remains dissolved, the final mass is too small.
Adding the precipitating reagent slowly helps prevent local excess concentrations. Constant stirring spreads the reagent through the liquid. A small excess is usually added to make the reaction go to completion, but a very large excess can introduce errors because unwanted ions may become trapped in the solid.
Fresh precipitates can be made of extremely small particles. These particles may pass through filter paper or hold large amounts of solution on their surfaces. To improve the solid, the mixture is often kept warm for a time after precipitation.
This step is called digestion. Small particles dissolve slightly and redeposit on larger particles. The larger crystals filter more easily and usually contain fewer impurities.
Students should notice that crystal size is not just about appearance. It affects how much material is recovered and how clean the recovered material is.
Filtration and washing need careful control. The solid is separated from the liquid using a filter, often filter paper, a glass frit, or a crucible designed for heating. The collected solid is washed to remove dissolved ions left between its particles.
Washing with pure water is not always best. Water can dissolve a little of some precipitates. A suitable wash liquid may contain a small amount of a harmless substance that reduces dissolution.
Too little washing leaves contamination and gives a mass that is too high. Too much washing can lose precipitate and give a mass that is too low.
Drying produces one of the most important checks in the method. The container and solid are heated, cooled in a dry container called a desiccator, then weighed. This cycle is repeated until successive masses are nearly unchanged.
A constant mass shows that moisture has been removed. Cooling in air can let the solid absorb water or carbon dioxide, so the desiccator matters. Handling the container with fingers can add oils or moisture.
In school laboratories, sulfate can be measured by forming barium sulfate, while chloride can be measured by forming silver chloride. These examples show how a measured mass becomes evidence about ions that were originally invisible in solution.
The calculation only works when the chemical formula of the dried solid is correct. Students should first write a balanced equation and inspect the mole ratio before using any masses. Units provide a useful error check.
Mass divided by molar mass gives an amount in moles. The balanced equation converts that amount into moles of the original substance. Converting back to mass gives the amount present in the sample.
This method teaches a wider lesson in chemistry. Careful experimental technique and a correct chemical model are both required for a trustworthy numerical result.
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.