Gravimetric Analysis of a Chloride Salt
Purpose: To illustrate typical techniques used in gravimetric analysis by determining quantitatively the chloride content in an unknown soluble salt.
Theory: AgCl(s) is a very insoluble solid, yet still does have some solubility. Because of these traits, the following reaction is able to occur: Ag+(aq) + Cl−(aq) → AgCl(s) .
This reaction is able to occur in both directions. When a soluble silver salt such as AgNO3 is added to a solution containing Cl−(aq) ion, then the added Ag+(aq) precipitates as AgCl(s) until all the chloride is consumed. For this particular reaction K sp = 1.6 x 10−10.
The reason why the solution is precipitated in an acidic medium become this causes the particle to dissolve faster, decreasing the time of reaction. This is the same reason why the solution is heated in order to coagulate because it speed up the particles inside the solution decreasing the time it needs to complete the reaction.
It is smart to use HNO3 when washing down the precipitate because when the AgCl(s) precipitates the particle decrease in size allowing it to pass through the filter paper. The precipitates are then heated in attempt to evaporate all the water until it precipitate is virtually entirely a solid.
Lastly, it is important that the AgCl(s) is placed in a dark space when not being used because of photodecomposition. This is the process that causes the solution to break down in the presence of light rays.
1) Gather unknown salt sample from TA and record its code number for future comparison. Verify there are two glass filter crucibles in the desiccator that were previously in the 110°C oven. Weigh out a sample of the unknown salt using an analytical balance anywhere between 0.1000g to 0.1500g and place in a labelled 250mL beaker. Calculate the approximate value of 0.1M of AgNO3 that will be needed for the precipitation and add 5mL in excess to ensure all chloride are precipitated. (Volume of 0.1M AgNO3 = [(Mass of sample x %Cl)/35.5]/0.1) Add 100mL of distilled water and 1mL of dilute (6M) HNO3. Stir solution with stirring rod until completely dissolved. Slowly add the calculated quantity of 0.1M AgNO3 and stir gently. Place beaker on hot plate to heat without boiling. Continue to stir gently. Check for completeness of precipitation by adding a few drop of AgNO3. If more AgNO3 appears, continue adding AgNO3 until precipitation is complete. Store solution in dark place once complete. Set up vacuum filtration apparatus.
Without touching the glass crucibles with bare hands, weigh them accurately to 0.0001g and record mass. Use crucible tongs to handle the glass crucibles. Decant the liquid through a weighed sintered glass filter then add a few drops of 0.01M HNO3 to the precipitate in the beaker. Stir cautiously and then decant through the filter. Repeat the 0.01M HNO3 washing, but this time transfer the precipitate and washings quantitatively onto the filter. Use a wash bottle to remove any remaining particles on the beaker walls. Continue washing the precipitate until the filtrate is essentially free of silver ion. To determine when the washing is complete, small amounts of washings are collected. By adding a few drops of HCl will detect any remaining silver. Washing is complete when no change occurs. Wash precipitate with three 5mL portions of precipitate. To obtain a constant weight, place both crucibles in the 110°C oven for approximately 30 mins. Then cool in desiccator and weigh. Repeat process until two successive readings have no more than a 1mg difference. Quickly return the used filter so it can be cleaned for reuse.
Observations, Data, and Calculations:
Unknown Salt Code Number: 355
Estimate of Percentage of Cl-= 55.00%
t= ± 0.2°C
m= ± 0.0001g
solution starts of white opaque colour
solution becomes a cloudy grey translucent colour with silver flakes on the bottom of the beaker solution becomes clear and...
Bibliography: 1. Burk, R.C., M. Azad, X. Sun, and P.A. Wolff. "Gravimetric Analysis of a Chloride Salt." Introductory Chemistry: Laboratory Manual.
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