Lab 16: Gravimetric Determination of a Precipitate

Purpose: The purpose of this lab is to use gravimetric analysis to obtain the formula of an unknown sulfate sample.  This is done by taking the analyte and making it insoluble first, then filtering and drying out all of the water. Once the mass of the precipitate is found, the moles can be found, and the percentage of the sulfate content is found and compared to other sulfates to determine what the unknown is.

Question: How can you use gravimetric analysis to obtain the formula of an unknown sulfate sample?

Background

Gravimetric analysis is one of the oldest and most accurate quantitative methods for determining the amount of an analyte in a sample.  Strategies usually involve transforming the analyte into a water insoluble from which precipitates out of solution and can be isolated by filtration and drying.  The final stage of the analysis is obtaining the mass.  From the measured mass and stoichiometric considerations, the mass, number of moles, and percentage of the sulfate content in the precipitate can be determined.

In the activity, the mass of a sample containing a sulfate of an alkali metal is measured and then dissolved in dilute hydrochloric acid.  The sulfate is isolated by precipitation with barium chloride.  The precipitate is digested in a heated solution to form coarser, easily filtered particles, and to purify the precipitate.  The precipitate is then collected by filtration, washed, dried, and its mass measured.  The amount of sulfate in the original sample can be calculated from the mass of the precipitate and its chemical composition.