What weight of sample should be taken for analysis so that exactly twice the weight in milligrams of precipitated BaSO4 is the percentage of SO3 in the original sample?
Answers
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 goes all the way to completion, then the mass of the substance in the original sample can be determined.
In this experiment, the percentage by mass of sulfate in an unknown sulfate salt will be determined by gravimetric analysis. First, a pre-weighed sample of the unknown sulfate salt will be dissolved in water. Next, an excess of aqueous barium chloride is added to the aqueous solution of the unknown salt. This will result in the precipitation of all the sulfate ions as barium sulfate:
Metal sulfate (aq) + Barium chloride (aq)⟶Barium sulfate (s) + Metal chloride (aq)
(7.1)
(7.1)Metal sulfate (aq) + Barium chloride (aq)⟶Barium sulfate (s) + Metal chloride (aq)
The barium sulfate precipitate is collected by filtration, dried and weighed. Since barium chloride is added in excess, and since the precipitation reaction goes to completion, we can assume that all of the sulfate is transferred from the original unknown sample to the precipitate. The mass of sulfate in the collected
BaSO
4
BaSO4
precipitate can be calculated via its percent composition. This also yields the mass of sulfate in the original unknown since:
mass of sulfate in the precipitate = mass of sulfate in the unknown sample (7.2)
(7.2)mass of sulfate in the precipitate = mass of sulfate in the unknown sample
Finally, using the mass of sulfate along with the initial mass of unknown used, the percentage by mass of sulfate in the original sample may now be calculated.
In order to obtain the best results, the collected
BaSO 4
BaSO4
crystals should be as large as possible. This considerably aids the filtration process (larger crystals are less likely to be pass through the filter paper), and it also minimizes the amount of impurities adsorbed onto the crystals (smaller surface area). In general, larger crystals are obtained when the rate of precipitation is as low as possible. The rate of precipitation is minimized by slowly adding the BaCl
2
BaCl2
solution to the aqueous mixture containing the unknown salt while continuously stirring the mixture. The rate of precipitation can be decreased even further by slightly increasing the solubility of the
BaSO 4
BaSO4
. This may be achieved by lowering the pH with 6 M HCl
HCl
and by increasing the temperature. The resulting decrease in the yield of the
BaSO 4
BaSO4
is insignificant.