Name: Date Completed: Lab Partner(s):
Lab # 4: Separation of a Mixture Lab Accelerated Chemistry 1 Objective You will be given a mixture containing sodium chloride (NaCl, table salt), benzoic acid (C7H6O2, a common food preservative), and silicon dioxide (SiO2, sand). Your goal is to separate the substances and determine the percent of each in the original mixture. Here is the separation scheme:
Mixture (NaCl, benzoic acid, sand)
Extract with 100°C H O 2 Decant
residue supernatant Sand (wet) NaCl, benzoic acid
Cool Filter evaporate H2O residue filtrate
Sand (dry) Benzoic acid (wet) NaCl (solution)
evaporate H2O
Benzoic acid (dry) Introduction Mixtures are not unique to chemistry; we use and consume them on a daily basis. The beverages we drink each morning, the fuel we use in our automobiles, and the ground we walk on are mixtures. Very few materials we encounter are pure. Any material made up of two or more substances that are not chemically combined is a mixture. The isolation of pure components of a mixture requires the separation of one component from another. Chemists have developed techniques for doing this. These methods take advantage of the differences in physical properties of the components. The techniques to be demonstrated in this laboratory are the following: Extraction. This uses a solvent to selectively dissolve one component of the solid mixture. With this technique, a soluble solid can be separated from an insoluble solid. Decantation. This separates a liquid from insoluble solid sediment by carefully pouring the liquid from the solid without disturbing the solid. Recrystallization. This involves dissolving a mixture of solids in a hot solvent, followed by cooling, and crystallization of the more insoluble material. Filtration. This separates a solid from a liquid through the use of a porous material as a filter. Paper, charcoal, or sand can serve as a filter. These materials allow the liquid to pass through but not the solid. Evaporation. You know what this is. AccelLab4-SepMixture 1
Pre-lab Questions 1. Of the methods listed for the separation of the components found in a mixture, which one would you use to remove mud from water?
2. Dry cleaners remove oil and grease spots from clothing by using an organic solvent called perchloroethylene. What separation technique are they using?
3. After Step 4 of the procedure, what type of mixture is present in Beaker 1 – a heterogeneous or homogeneous mixture? Explain.
4. Why is boiling water used in Step 7 of the procedure to wash the sand?
5. After step 7, what type of mixture is present in Beaker 2 – a heterogeneous or homogeneous mixture? Explain.
Procedure 1. Use a mortar and pestle to grind your sample and remove any lumps. 2. Mass a clean, dry 250-mL beaker to the nearest centigram (0.01 g); this is Beaker 1. Using a pencil, label the beaker with your initials on the white labeling spot. Record the mass. 3. Transfer your ground sample into the massed beaker. Mass the beaker + sample; record in the data table. Wash the mortar and pestle with warm water and a brush; dry Decanting and replace. 4. Add about 75-mL of tap water to the beaker and place on the hot plate. While stirring occasionally with your stirring rod, bring the mixture to near boiling. This only takes a setting of about half-way on the dial; no need to crank the hot plate to max setting—a higher setting will ultimately reach a higher temperature, but the rate of heating is the same. At the boiling temperature, all benzoic acid and sodium chloride should be in solution (there should be no white solids visible). If necessary, add ~10 mL increments of water until dissolution is complete. Too much water will lower your final percent recovery. 5. In addition, place about 100 mL of tap water in a clean 250 mL beaker on the hot plate. You will use the boiling water in step 7; share this water with your bench-mates. 6. Decant the supernatant liquid from your hot mixture into a second 250-mL beaker, guiding the solution by pouring down a glass rod. Use hot hands to hold the beaker of hot solution. Do not let the sand get into the second beaker.
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7. To make sure that all the soluble materials are separated from the sand, wash the sand with about 20-mL of boiling water and decant the washings into the second beaker. If necessary, use a second rinse with boiling water to get all the soluble material dissolved. 8. Prepare an ice/water bath in a 1-L beaker. Place Beaker 2, containing the water solution of benzoic acid and salt, into the ice bath and let it cool. Be careful that the beaker doesn’t tip/sink into the ice bath.
9. While Beaker 2 is cooling, warm Beaker 1, containing the sand, on the hot plate. The sand tends to splatter if heated too rapidly. You may cover the beaker with a watch glass (mass it first!) to minimize losses. When the sand is completely dry, allow the beaker to cool to room temperature and mass Beaker 1+sand; record.
10. Mass a 12.5-cm filter paper; record the mass in the Data Table. Write your initials on the filter paper with a pencil. Fold the filter paper as shown.
11. Assemble the apparatus for gravity filtration as shown. Place the filter paper snugly in funnel and wet slightly so it adheres to the funnel. Choose an appropriate-sized funnel so the filter paper does not extend over the funnel edge. Position a beaker under the funnel. Pour the cold recrystallized benzoic acid mixture through the filter, being careful to not fill the funnel past the leave of the filter paper. Transfer wet solids into the funnel with a scoopula. Rinse traces of solid into the funnel with small amounts of cool water from your wash bottle. When filtration is complete, carefully (wet filter paper tears easily!) remove the wet filter paper and solids with a spatula/forceps and place on a watch glass labeled with your initials. Place the watch glass on the teacher’s bench; your crystals will be set aside for drying.
12. Clean up your work area. Dispose of the sand in the trash. Dispose of the filtrate (containing dissolved sodium chloride) down the drain. Wash your hands.
Calculations Due to time constraints, will not recover NaCl(s) from the solution after collecting the benzoic acid crystals. We will calculate % sand and % benzoic acid in the original mix based on a 100% yield (no losses due to manipulations, lab technique); calculate % NaCl by the difference. Example calculation. A lab team isolated the following from a sample of 6.00 g:
2.10 g sand % sand = 2.10g sand/6.00g sample × 100 = 35.0% 1.80 g benzoic acid % benzoic acid = 1.8g benzoic acid/6.00g sample × 100 = 30.0% % NaCl = (6.00 – 2.10 – 1.80)/6.00 × 100 = 35.0%
Use the following formula to calculate % error:
| accepted value – experimental value | % error = × 100 accepted value
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You may need the following graph for an analysis question.
Solubility, g/100 g water 50 40
30 Benzoic Acid NaCl 20
10 g solute/100 g water 0
0 20 40 60 80 100 Temperature, °C
Observations Appearance of original mixture:
Appearance of recrystallized benzoic acid:
Data
1. Mass of Beaker 1 g
2. Mass of Beaker 1 + sample g
3. Mass of sample g
4. Mass of Beaker 1 + dry sand g
5. Mass of recovered sand g
6. Mass of filter paper g
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7. Mass of dry filter paper + benzoic acid g
8. Mass of recovered benzoic acid g
Calculations 1. Calculate the percent of each component in the mixture. Show your calculations.
Percent sand:
Percent benzoic acid:
Percent NaCl:
2. Obtain the percent composition of the teacher’s mixture. Calculate the percent error in your results. Show your calculations.
Sand % error:
Benzoic acid % error:
NaCl % error:
Analysis 1. If all the teams in the class received a sample of the same mixture, each team will not obtain the same result for its percentage composition. Give two main types of reasons (hint: read the remaining questions).
2. If the purpose of the experiment was to determine the percentage composition of a compound, rather than a mixture, requiring chemical separation methods, should each team obtain the same result for the percentage composition? Why?
3. Explain in your own words why the benzoic acid was able to be separated from the NaCl by cooling the solution.
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4. A lab team rushed through the lab and did not sufficiently cool the recrystallizing mixture of benzoic acid/water. How would this affect their value for the % of benzoic acid in the originial mixture? Explain.
5. Explain in your own words why adding too much water to completely dissolve the benzoic acid in the beginning of the lab would have decreased the amount of benzoic acid recovered in the end.
6. If your result for % sand was higher than in the original mix, what are possible sources for this experimental error?
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