GRAVIMETRIC ANALYSIS OF A CHLORIDE SALT (2013)
Report By: ____________ Lab Partner: ___________ Group: CHEM 1001 Tuesday AM, Group H
Purpose: This lab was conducted in order to determine the content of chloride in an unknown salt, using gravimetric analysis.
Theory: The salt chloride content is easy to find because it is slightly soluble, making it possible to turn it into a precipitate. A precipitate reaction can be done using silver to isolate the specific ion.
Ag+(aq) + Cl-(aq) --> AgCl(s)
Despite AgCl(s) being highly insoluble, some of the substance will still dissolve. This dissolved portion can be calculated.
AgCl(s) => Ag+(aq) + Cl-(aq) Ksp = [Ag+(aq)]·[Cl-(aq)] = 1.6 x 10-10
Since Ksp is so small the dissolved are to be considered negligible. Both ions will precipitate out very quickly to AgCl when the AgNO3 is added to the solution with the Cl- ions until all of these ions are consumed. Small amounts of salt will still remain in the solution because the maximum solubility to be reached when there is no longer excess Ag+. The remaining Cl- can be calculated still.
With no excess Ag+, [Ag+] = [Cl-]; let both = x
Ksp = [Ag+(aq)]∙[Cl-(aq)] = 1.6 x 10-10 [x]∙[x]=1.6 x 10-10 x =1.3 x 10-5
Due to the speed of formation, it precipitates as a colloid, and the particles remain very small due to the lack of time to crystallize. The colloid must be heated in an acid media to form larger, crystalline pieces so it can be poured into the crucible without passing through the filter. The acid media also prevents the new crystalline pieces from going back to a colloid state. Another cause of the fast reaction is the trapping of other ions. The acid media also resists any interference of any anions, such as CO3 2-, that may be able to for precipitate with silver ions.
Photodecomposition can occur in the precipitate (AgCl(s) => Ag(s) + (1/2)Cl2(g)). This reaction mainly occurs on the surface of the precipitate because it is most exposed to light, making the effects not too high. With significant excess Ag+, the decomposition can lead to CIO3 being formed (3Cl2(g) + 9H20(l) + 5Ag+(aq) => 5AgCl(s) + CIO3-(aq) + 6H3O+(aq)).
The amount of precipitate lost during the 100 mL distilled water rince can be calculated using the Ksp value.
n=Ksp x 0.1 L n= 1.6 x 10^-11 mols
m= (1.6 x 10^-11 mol)(143.32 g/mol) m= 2.3 x 10^-9 g Therefor a maximum of 2.3x10^-9 g can be lost during the rinse.
All of the following procedures where carried out individually by both laboratory partners. The unknown salt sample was obtained and its number recorded. An analytical balance was used to weigh out by difference a sample of the salt weighing just over 0.1000g and the sample was then placed in a numbered 250mL beaker. The volume of 0.1 M AgNO3 needed for the precipitation was calculated between waiting times for the analytical balance. 100 ML of distilled water and 1 mL of dilute HNO3 was then added to the beaker with the salt sample and stirred. The necessary quantity of AgNO3 was slowly added to the beaker as well while stirring the contents of the beaker. The beaker was then placed onto a hot plate to heat up, and was heated and gently stirred until the contents became fairly translucent in comparison to its original state. Drops of silver nitrate were added to check for completeness.
The beaker was then placed into a drawer, out of any light source, to cool. The crucible was labeled and weighed in the analytical balance. The crucible was then used to set up a vacuum filtration apparatus, and the liquid in the beaker was poured through the crucible while the precipitate remained in the beaker. 0.01 M HNO3 was added to the beaker and the beaker was gently swirled to wash the precipitate. The washings were then poured through the crucible and the precipitate remained in the beaker. The same HNO3 wash was conducted again, except both the washings and the precipitate were poured onto the filter. More of the HNO3 was poured over the precipitate to wash it. After the removal of the crucible, the flask in the vacuum collection was cleaned and set back up with the crucible.
Another wash of the precipitate was done with the HNO3, and the new washings in the flask were transferred into a test tube for a TA to test for turbidity. Once this test passed, the precipitate was washed with three 5 mL portions of acetone using the filtration system. The crucible was dried by placing it into an oven for about half of an hour. After drying the crucibles were cooled then weighed using the analytical balance.
Description Precipitation Completion Test Completeness of Washing Test Precipitate Description Sample White, fine granular powder. Positive Positive Precipitate was a purple/white powdered substance Partners Sample White, fine granular powder. Positive Positive Precipitate was a purple/white powdered substance
Values Partners Values Weight of Sample 0.1313 g (+/- 0.0001 g) 0.1484 g (+/- 0.0001 g) Weight of Precipitate 0.2746 g (+/- 0.0002 g) 0.3134 g (+/- 0.0002 g) Volume of AgNO3 20.3 mL 27.99 mL Oven Temperature 108 C (+/- 0.2 C) 108 C (+/- 0.2 C) Drying Time 28 min 28 min Cooling Time 5 min 5 min Actual % of Cl 53.35% 53.35%
Sample 1 Partners Sample n= (0.2746 g) / (107.8682+35.453)g/mol n= 0.0019 mol
m= 0.0019 mol x 35.435 g/mol m= 0.0674 g
%m= (0.0674 g/ 0.1313 g) x 100% %m= 51.33%
error= ((51.33%-53.35%)/(53.35%))x100 error= -3.79% n= (0.3134 g) / (107.8682+35.453)g/mol n= 0.0022 mol
m= 0.0019 mol x 35.435 g/mol m= 0.0780 g
%m= (0.0674 g/ 0.1313 g) x 100% %m= 52.56%
error= ((52.56%-53.35%)/(53.35%))x100 error= -1.48%
Avg= (51.33%+52.56%)/(2) Avg= 51.95%
Rel. Spread= ((52.56%-51.33%)/(51.95%))x1000 ppt Rel. Spread= 23.7 ppt
Discussion: This experiment proved to be very accurate. Looking at the relative error for both samples, the value is very small. This information proves the use of proper laboratory techniques were prominent and the procedure was followed carefully. The results were not perfect, do to uncertainties and human error. Both percent values were lower than the actual value.
A reason for this is the silver chloride decomposing back to its ions due to the exposure of light to the precipitate. When this exposure of light occurs the precipitate appears to turn violet, which both precipitates did. The decomposition in light plays a huge factor in a lower yield. Other reasons could be inaccurate use of the analytical balance, leaving small remnants of the precipitate in the beaker and also small amounts of precipitate may have passed through the crucible.
Conclusions: The salt sample #360 was calculated to have an average value of 51.95% Cl, when compared to the actual value of 53.35% is very similar. The accuracy of this experiment was excellent with errors of -3.79% and -1.48%, and a decent precision with a relative spread of 23.7 ppt.
Burk, R. C., Azad, M., Sun, X., & Wolff, P. A. (2013). Introduction chemistry laboratory manual. (CHEM 1001/1002 ed., pp. 57-59). Ottawa: Carleton University.