Lab #1–Separation of Mixtures– Fractional Crystallization
Background
All separation techniques are based on differences in physical properties; furthermore, the bigger the difference in properties, the easier the separation. In this procedure you will be separating a mixture of three solids - sand, copper(II)sulfate, and potassium nitrate. The sand can be easily separated from the other two based on water solubility: sand is completely insoluble in water (which explains its ubiquitous presence on beaches), while the salts CuSO4 and KNO3 are soluble in water. We can therefore add as much water is needed to completely dissolve both the CuSO4 and KNO3 without worrying about dissolving any of the sand.
Separating the CuSO4 and KNO3 is trickier as they are both water soluble. To accomplish the trickier separation we will be taking simultaneous advantage of two differences. First, the CuSO4 is found in much smaller quantity in the samples, and so we can keep it dissolved with a minimum amount of solvent without also dissolving a significant part of the KNO3. The second difference in properties is more subtle, and has to do with the temperature dependence of solubility. KNO3 is far more soluble in hot water than in cold, while CuSO4 is only slightly more soluble in hot water.
Procedure
Separating the sand
Obtain approximately 25 grams of the sample mixture and record its mass. Combine the solid sample with about 50 mL of water in a 150 mL beaker. Set up a ring stand with ring and wire gauze to support the beaker and gently warm the solution to about 40ºC, stirring constantly.
When the blue CuSO4 and white KNO3 appear to be completely dissolved, use the suction filtration apparatus demonstrated by your instructor, with a preweighed piece of filter paper, to filter the sand /solution mixture. Use a rubber policeman (at the end of a stirring rod) to transfer as much of the sand as possible to the Buchner funnel. Remove the funnel and transfer the blue filtrate in a clean 250 ml beaker. Reassemble the suction apparatus and rinse the sand with distilled water to remove any residual CuSO4 and KNO3. Use the suction to help dry the recovered sand. Transfer the filter paper and wet sand to a watch glass or glass plate and store it in your drawer to dry overnight. Be sure to weigh the dry sand and filter paper the next time you are in the lab.
Removing the CuSO4 impurity from the KNO3
Add 15 drops of 6 M HNO3, nitric acid, to the blue filtrate you saved in the previous step. The HNO3 helps keep the CuSO4 in solution through the remainder of the procedure. Gently heat the solution to boiling, controlling the rate of heating by moving the Bunsen burner in and out of position below the beaker. Continue to boil gently until white KNO3 crystal appear in the hot solution; the solution may look cloudy at this point. Add another 5 mL of distilled water and scrape any crystals from the walls of the beaker back into the solution. Reheat if necessary to redissolve all solids. (***This may be a good stopping point*** Store the solution in your drawer in a beaker covered with an inverted watch glass.)
Cool the solution to room temp. in a tap water bath, then cool to about 0ºC in an ice bath. You should see copious amounts of white crystals forming. At this point, we want the white KNO3 to crystallize out of solution, while the blue CuSO4 impurity remains in solution. If the solution becomes mostly solid, you may want to add another 5 mL of water to keep the CuSO4 in solution.
The following steps depend on maintaining low temp. throughout the filtration. Working quickly to avoid rewarming is essential. Assemble the suction filtration apparatus and chill the Buchner funnel by placing 4 or 5 ice cubes in it and drawing distilled water over the ice cubes and through the funnel. Discard the ice and place a preweighed piece of filter paper in the funnel. Remove the sample mixture from the ice bath and use the rubber policeman to filter the solid/solution slurry. Press a second clean, preweighed piece of filter paper on top of the solid in the funnel and continue applying suction to help dry the sample. Store the sample and filter papers in your drawer on a watch glass or glass plate overnight. Weigh and record the mass the next time you are in lab.
Analyzing the Purity of the Recovered KNO3
Weigh a 0.5 g sample of your recovered KNO3 and dissolve it in 3 mL of water in a 50 mL beaker. Add 3 mL of 6 M NH3. This will form a copper(II)tetraammine complex with an extraordinary dark blue color. Pour some of this blue solution into a small test tube, and compare it to the standards provided by your instructor. Record the % Cu indicated by the standard that is closest in color to your sample.
Recrystallizing the Recovered KNO3
Weigh the remainder of your more or less purified KNO3 sample in a 150 mL beaker. Use the mass of your sample and the solubility graph provided by your instructor to determine the amount of 100ºC water needed to dissolve the entire sample. Add three times that amount of room temp distilled water, and stir to dissolve. Cool the mixture to 0ºC in an ice bath. Cold filter the resulting slurry follow the procedure used above. When the sample is dry, weigh and record its mass. Analyze the Cu impurity as you did with the first crystallization. Record the % Cu of the recrystallized sample.
Prelab summary
Procedural Tips: When using the suction filtration apparatus, following the sequence: Water on-connect hose-filter-disconnect hose-water off. This prevents tap water backflow. Remember water-hose-hose-water.
Speed is of the essence in cold filtration.
Always preweigh filter papers. It’s also a good habit to always preweigh and record the mass of empty beakers.
Be sure to transfer the blue filtrate to a separate beaker before rinsing the sand. Failure to do so will add boatloads of water to your filtrate, requiring more time than you have to boil away.
Sidearm filter flasks are expensive ($20’ish) and top-heavy. Always have clamped to a ring stand.
Safety: All 6 M acid and base solutions are dangerous. Your eyes are protected ALWAYS by goggles. Report any skin contact to your instructor as soon as you have rinsed the affected area with large quantities of tap water. Report any significant spills to your instructor, who will direct you in your cleanup efforts.
Read labels carefully (as always). This procedure features HNO3 and NH3, which are easily confused.
Use common sense around open flames. Hair is especially flammable.
Never use a thermometer as a stirring rod.
Disposal: All solid waste should be put in a trash can. All liquid waste can be poured down the sink drain.
Data and Calculations
1st Crystallization / Recrystallized SampleMass of sample at start of crystallization
Mass of dry Sand recovered / X
% dry Sand in original sample / X
Mass of purified KNO3 recovered
% KNO3 in original sample
% CuSO4 in purified sample
PreLab Questions
Use the solubility graph provided by the instructor to answer the following questions. Show work as needed.
1. How many grams of KNO3 will dissolve in 100. g H2O at 100ºC?
2. How many grams of water are required to dissolve 25 g of KNO3 at 100ºC? (Hint: Your answer to #1 is handy conversion factor)
3. How many grams of water are required to dissolve 2.0 g CuSO4 at 100ºC?
4. How many grams of 100ºC water are required to dissolve a mixture of 25 g KNO3 and 2.0 g CuSO4. Assume the solubility of one substance is not affected by the presence of a second substance. In other words, water can dissolve both simultaneously.
To the solution created as described in #4, 10 g of water are added and the solution is cooled to 0ºC.
5. How many g KNO3 remain in solution?
6. How much KNO3 crystallizes out?
7. How much CuSO4 crystallizes out?
8. What percentage of the KNO3 in the sample is recovered?
PostLab Questions
1. Identify three intended aspects of the procedure that affect the purity and yield of KNO3. For each of the aspects: (1) Describe the aspect, (2) Describe how yield is affected, (3) Explain why yield is affected,
(4) Describe how purity is affected, and (5) Explain why purity is affected.
(A) Aspect #1:
How is yield affected:
Why is yield affected:
How is purity affected:
Why is purity affected:
(B) Aspect #2:
How is yield affected:
Why is yield affected:
How is purity affected:
Why is purity affected:
(C) Aspect #3:
How is yield affected:
Why is yield affected:
How is purity affected:
Why is purity affected:
2. In general, what is the relationship between the purity of the recovered KNO3 and the yield of recovered KNO3?