Experiment Overview

Gravimetry is an analytical technique used to determine the amount of a substance by converting it into a stable, insoluble compound that can be weighed accurately. This method is highly precise and widely used for quantitative chemical analysis.

What This Experiment Demonstrates

• Quantitative determination of ions through precipitation
• Purification and isolation of solid compounds
• Calculation of concentrations using mass measurements

Apparatus Required

• Beaker
• Filtration setup (Büchner funnel, filter paper, vacuum flask)
• Weighing balance
• Glass rod
• Evaporating dish
• Desiccator

Chemicals Required

• Sample solution containing the analyte
• Precipitating reagent (e.g., barium chloride for sulfate ions)
• Distilled water
• Wash liquids (e.g., dilute acid, water or alcohol, depending on the substances)

Theory

In gravimetry, the analyte is converted into a pure, stable, and insoluble compound through a precipitation reaction. The precipitate is then separated, purified, dried, and weighed. The mass of the precipitate is used to calculate the amount of analyte present in the original sample.

For example, when determining sulfate ions, the precipitation reaction is:

\[\ce{Ba^{2+} + SO4^{2-} -> BaSO4(s)}\]

After the precipitate is washed, dried, and weighed, the moles of the analyte are calculated using the molar mass of the precipitate:

\[n(\ce{SO4^{2-}}) = \frac{m(\ce{BaSO4})}{M(\ce{BaSO4})}\]

Method

1. Prepare the sample solution containing the analyte.
2. Add a suitable precipitating reagent slowly with stirring to form the insoluble compound.
3. Allow the precipitate to settle and filter it using a Büchner funnel and vacuum filtration.
4. Wash the precipitate thoroughly to remove impurities.
5. Dry the precipitate in an oven or desiccator to constant mass.
6. Weigh the dried precipitate accurately.
7. Calculate the amount of analyte in the original sample using stoichiometry.

Observations

A dense, crystalline precipitate forms upon addition of the precipitating reagent. Washing ensures removal of soluble impurities, and drying gives a stable solid suitable for accurate weighing.

Calculations

• Calculate the moles of analyte using the mass and molar mass of the precipitate.
• Determine the concentration or percentage of the analyte in the sample solution.
• Use appropriate stoichiometric relations if more than one species is involved.

Result

The precise amount of analyte in the sample is determined by measuring the mass of the purified precipitate formed during the gravimetric procedure.