Gravimetric Analysis of SULFATE as Barium Sulfate.
* To learn the techniques associated with gravimetric analysis. * To use stoichiometry to calculate the percentage by mass of sulfate in an unknown sulfate salt. Background
Gravimetric analysis is a quantitative method for accurately determining the amount of a substance by selective precipitation of the substance from an aqueous solution. The precipitate is separated from the remaining aqueous solution by filtration and is then weighed. Assuming that the chemical formula for the precipitate is known and that the precipitation reaction is stoichiometric (goes to completion) the mass of the substance in the original sample can be determined. In this experiment you will determine the percentage (by mass) of sulfate in an unknown sulfate salt by gravimetric analysis. First you will dissolve a measured mass of the unknown salt in water. Next you will add an excess of aqueous barium chloride to the aqueous solution of the unknown salt. This will result in the precipitation of the sulfate as barium sulfate.
BaCl2(aq) + M2SO4(aq) →BaSO4(s) + 2 MCl(aq)
(assuming +1 cation)
BaCl2(aq) + MSO4(aq) →BaSO4(s) + MCl2(aq)
(assuming +2 cation) The barium sulfate precipitate is collected by filtration, ignited and weighed. The number of moles of sulfate can be determined from the mass of the barium sulfate. Since barium chloride is added in excess, and since the precipitation reaction is assumed to go to completion, the number of moles of sulfate recovered in the precipitate can be assumed to be equal to the number of moles of sulfate in the original sample allowing for the calculation of the percentage by mass of sulfate in the original sample. In order to obtain the best results the BaSO4crystals should be as large as possible. This facilitates filtration and washing of the crystals and the decreased surface area minimizes the amount of impurities adsorbed onto the crystals. Generally the...
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