Separation of the Components of a Mixture Background 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: 1. Sublimation. This involves heating a solid until it passes directly from the solid phase into the gaseous phase. The reverse process, when the vapor goes back to the solid phase without a liquid state in between, is called condensation or deposition. Some solids which sublime are iodine, caffeine, and para-dichlorobenzene (mothballs). 2. 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. 3. Decantation. This separates a liquid from insoluble solid sediment by carefully pouring the liquid from the solid without disturbing the solid (Fig. 1). Figure 1. Decantation 4. 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 (see Fig. 4 in the Procedure section). 5. Evaporation. This is the process of heating a mixture in order to drive off, in the form of vapor, a volatile liquid, so as to make the remaining component dry. The mixture that will be separated in this experiment contains three components: naphthalene (C10H8), common table salt (NaCl) and sea sand (SiO2). The separation will be done according to the scheme in Fig.2 by: 1. Heating the mixture to sublime the naphthalene, 2. Dissolving the table salt with water to extract, and 3. Evaporating water to recover dry NaCl and sand. Mixture (Naphthalene, NaCl, Sand) Figure 2. Separation Heat to 250 oC Scheme Sublimate Residue Naphthalene NaCl, Sand Extract with H2O Filter liquid residue NaCl Sand (solution) (wet) evaporate evaporate H2O H2O NaCl Sand Objectives: 1. To demonstrate the separation of a mixture. 2. To examine some techniques for separation using physical methods. WEAR YOUR SAFETY GLASSES WHILE PERFORMING THIS EXPERIMENT Procedure 1. Obtain a clean, dry 150-mL beaker and carefully weigh it to the nearest 0.001 g. Record this weight for beaker 1 on the Report Sheet (1). Obtain a sample of the unknown mixture from your instructor. With the beaker still on the balance, transfer all of the unknown mixture into the beaker. Record the weight of the beaker with the contents to the nearest 0.001 g (2). Calculate the exact sample weight by subtraction (3). 2. Place an evaporating dish on top of the beaker containing the mixture. Place the beaker and evaporating dish on a wire-gauze with a tripod as shown in Fig. 3. Place ice in the evaporating dish, being careful not to get any water on the underside of the evaporating dish or inside the beaker. Figure 3. Assembly for sublimation 3. Carefully heat the beaker with a Bunsen burner, increasing the intensity of the flame until vapors appear in the beaker. A crystalline solid should collect on the underside of the evaporating dish. After 10 min. of heating, remove the Bunsen burner from under the beaker. Carefully remove the evaporating dish from the beaker and collect the solid by wiping it off with a paper towel or scraping it off with a spatula. Drain away any water from the evaporating dish and add ice to it, if necessary. Stir the contents of the beaker with a glass rod. Return the evaporating dish to the beaker and apply the heat again. Continue heating and wiping/scrapping off solid until no more solid collects. Discard the naphthalene into a special container provided by your instructor. 4. Allow the beaker to cool until it reaches room temperature. Weigh the beaker with the remained solid (4). Calculate the weight of the naphthalene that sublimed by difference (5). 5. Add about 25 mL of distilled water to the solid in the beaker. Heat and stir for five minutes. It is NOT necessary to boil this solution. 6. Weigh a second clean, dry 150-mL beaker with 2 or 3 boiling chips (glass beads), to the nearest 0.00 1 g (6). NOTE: weigh the beaker and the boiling chips together. 7. Assemble the apparatus for gravity filtration as shown in Fig.4. Figure 4. Gravity filtration Funnel tip should touch the beaker in such a way that filtrate will run down the wall of the beaker. 8. Fold a piece of filter paper following the technique shown in Fig.5 Figure 5. Steps for folding a filter paper for gravity filtration 9. Wet the filter paper with water from squirt-bottle and adjust the paper so that it lies flat on the glass of the funnel. 10. Position the second beaker under the funnel 11. Pour the mixture through the filter. Collect all the liquid (called the filtrate) in beaker 2. 12. Any remaining sand may be removed by squirting some water from the squeeze-bottle while holding the beaker over the funnel. Rinse beaker twice with about 5mL of water. Pour rinsings into the funnel. 13. Place beaker 2 and its contents on wire gauze with an iron ring and ring stand assembly. Begin to heat gently with a Bunsen burner. Control the flame in order to prevent boiling over. As the volume of liquid is reduced, solid sodium chloride will appear. Reduce the flame to avoid bumping of the solution and spattering of the solid. When all of the liquid is gone, cool the beaker to room temperature. Weigh the beaker, chips, and the solid residue to the nearest 0.001 g (7). Calculate the weight of the recovered NaCl by subtraction (8). 14. a. Carefully remove the filter paper containing the sand. Place the sand and filter paper in the pre-heated drying oven ( 90-100 0 C ). Wait 10-15 minutes and check to see if sand and filter paper are dry. b. If dry, CAREFULLY remove from oven and allow the sand to cool to room temperature. Weigh the filter paper and the sand to the nearest 0.001 g (9). c. Shake off all of the sand and weigh your filter paper. (10) Calculate the weight of the recovered sand by subtraction (11). 15. Calculate: a. Percentage yield using the formula: % yield = (g of solid recovered/g of initial sample) *100 b. Percentage of each component in the mixture by using the formula: % component = (g of comp. isolated/g of initial sample)*100 Sample calculation A student isolated the following from a sample of 1.132 g: 0.170 g of naphthalene 0.443 g of NaCl 0.499 g of sand 1.112 g solid recovered The student calculated the percentage yield and percentage of each component as follows: % yield = (1.112 g solid recovered) / (1.132 g original sample)*100 % C10H8 = (0.170 g naphthalene) / (1.132 g original sample)*100 % NaCl = (0.443 g NaCl) / (1.132 g original sample)*100 % sand = (0.499 g sand) / (1.132 g original sample)*100 Name ___________________________ Date ______________ Partner ________________________ Section ___________ Data Sheet: Separation of a Mixture Unknown # ____________________ 1. Weight of beaker 1: __________ g 2. Weight of beaker 1 and mixture: __________ g 3. Weight of mixture: (2) — (1) : __________ g 4. Weight of beaker 1 and solid after sublimation: __________ g 5. Weight of naphthalene: (2) – (4) __________ g 6. Weight of beaker 2: __________ g 7. Weight-of beaker 2 and NaCl: __________ g 8. Weight of NaCl: (7) — (6) : __________ g 9. Weight of filter paper & sand: __________ g 10. Weight of filter paper alone: __________ g 11. Weight of sand: (9) — (10) : __________ g Calculations 12. Weight of recovered solids: __________ g (5)+ (8) + (11) 13. Percentage yield (percentage of solids recovered): __________ % [(12)/(3)] * 100 14. Percentage of naphthalene: __________ % [(5)/(3)] * 100 15. Percentage of NaCl: __________ % [(8)/(3)] * 100 16. Percentage of sand: __________ % [(11)/(3)] * 100 Name ___________________________ Section __________________________ Post-lab Questions: Separation of a Mixture 1. A student started this experiment with a mixture weighing 2.345 g. After separating the components, a total of 2.765 g of material was recovered. Assume that all the weighings and calculations were done correctly. How do you account for the apparent increase in weight of the recovered material? 2. Ice cubes stored in the freezer compartment of a refrigerator for a long period of time lose their shape and shrink in size. Account for this observation. 3. The weight of naphthalene in your sample could be determined either by difference (as in this experiment) or by directly weighing the amount of solid collected on the evaporating dish. Which method is a more accurate method? Explain your answer. 4. A sample of French fried potatoes weighing 100.0 g was extracted with the volatile organic solvent hexane. After separation and evaporation of the hexane, 6.25 g of cooking oil was recovered. What was the percent oil in the potatoes? Show your calculations. 5. Dry cleaners remove oil and grease spots from clothing by using an organic solvent called perchloroethylene. What separation technique do the cleaners use? 6. From an 11.562-g sample containing sodium chloride, naphthalene, and sand, the following were recovered: 3.642 g sodium chloride, 1.564 g naphthalene, and 5.921 g sand. Calculate the percentage of each substance in the sample and the total percentage of sample recovered. If your calculations show less than 100% recovery, what could account for the difference? Show all your work. Name ___________________________ Section __________________________ Pre-lab Assignment: Separation of a Mixture 1. Of the 5 methods listed for the separation of the components found in a mixture, which one would you use to remove mud from water? 2. Can any of the methods listed in the Background section be used to separate the elements found in a compound? Explain. 3. What separation technique(s) is (are) used when making a cup of tea by soaking a tea bag in hot water? 4. para-Dichlorobenzene can be used effectively as a moth repellent. What property of this compound allows it to be used in mothballs for clothes protection?