Gravimetric Analysis: Key Concepts
Gravimetric analysis is the quantitative isolation of a substance by precipitation and weighing
of the precipitate. It employs a double displacement (precipitation) reaction.
An analyte is the substance to be analyzed.
A precipitating reagent is the reactant used to precipitate the analyte.
Procedure:
Weigh the sample to be analyzed and heat to remove excess water.
Prepare a solution of the sample. This dissociates the ions of the analyte.
Add sample to an excess of the precipitating reagent to precipitate the analyte
Filter the mixture to separate the precipitate from the solution
Wash the precipitate to remove any impurities
Dry the precipitate by heating to remove water
Cool the precipitate in a desiccator to prevent the precipitate absorbing moisture from the air
Weigh the cooled precipitate
Repeat the drying and weighing process until a constant mass for the precipitate is achieved
Calculate the percent by mass of analyte in the sample
General calculation of the percent by mass of analyte in a sample:
Write the balanced chemical equation for the precipitation reaction
Calculate the moles of precipitate: moles = mass ÷ molecular mass
Calculate moles of analyte from the balanced chemical equation using the mole
ratio of analyte: precipitate
Calculate mass of analyte: mass = moles x molecular mass
Calculate percent by mass of analyte in sample: (mass analyte ÷ mass sample) x 100
Common Sources of Error:
Incomplete precipitation results in a value for the percentage of analyte in the sample that is
too low.
Incomplete drying of the sample results in a value for the percentage of analyte in the sample
that is too high.
Other ions in the sample may also be precipitated resulting in a value for the percentage of
analyte in the sample that is too high.
