Separation of a Mixture: Determining the Percent Composition of Ammonium Chloride, Salt, and Sand OBJECTIVES: Use physical properties to separate the components of a mixture. Calculate the percent composition of a mixture. INTRODUCTION: Most materials found in nature are mixtures of different substances. A mixture is the combination of two or more substances in which each substance retains its own chemical identity. Mixtures that have varying compositions throughout are heterogeneous mixtures (e.g. wood, granite, soil and pizza); mixtures that are uniform in composition and properties are homogeneous mixtures (e.g. clear iced tea, many chemical solutions and air). While pure substances have fixed compositions, and can only be separated by chemical means, physical mixtures can have varying compositions and can be separated with physical processes. Separation of physical mixtures is often the first step in refining products whether for lab or industrial purposes. Most solid substances, when heated, undergo a phase transition from the solid phase to the liquid phase, a process known as melting. However, some substances go from the solid phase directly to the gas phase, a process known as sublimation. (A familiar example is frozen carbon dioxide, or dry ice. If you put a chunk of dry ice on a tabletop, the dry ice does not melt, but the chunk gets smaller and smaller as the particles in the solid warm up and go directly into the gas phase.) Ammonium chloride sublimes when heated, which gives us a convenient way to separate ammonium chloride from salt and sand. By knowing the weight of the sample before and after heating, the mass of the ammonium chloride in the sample can be determined. Another physical property can be exploited to separate the sodium chloride from the sand: sodium chloride is soluble in water, while sand is not. (As you know if you’ve ever been to a beach.) Hot water will be added to the mixture to dissolve the sodium chloride, and the salt water is removed by decanting it away from the sand. The sand is then dried over a low flame so that its mass can be determined. Once the weights of each component are known, the percent composition of the mixture is determined by dividing the weight of each component by the total weight of the sample and multiplying by 100. PRE-LAB QUESTIONS: 1. 2. 3. 4. Copy the table following the procedure into your lab notebook. Why should you never weigh a hot object? Define the process of sublimation? Suggest two SAFE ways to determine whether a colorless liquid is pure water or a salt solution. PROCEDURE: A. Weighing the Unknown Sample. 1. The overall process used in this experiment is noted in figure 1 to the right. Obtain a sample of an unknown mixture of ammonium chloride and sand from the stockroom and record the sample number on the lab report . 2. Clean an evaporating dish, warm it over a Bunsen burner to dry, allow it to cool, and weigh it to the accuracy of the balance. Handle the clean dish with tongs, not your hands. Add between two to three grams of your unknown mixture to the dish, weigh it again, and calculate the mass of the unknown mixture. Mixture: NH4Cl NaCl SiO2 Heat to sublime NH4Cl (mass loss) Remaining NaCl SiO2 Extract NaCl with water, dry & mass Dry SiO2 residue and mass Figure 1 – separation process process=s B. Separation of Ammonium Chloride from the Mixture by Sublimation. 3. Place the evaporating dish containing the mixture hot plate in the fume hood. Heat carefully at first to avoid any spattering, and increase the heat after a couple of minutes. After heating strongly for 8-10 minutes, stir the contents with a glass stirring rod. Heat the sample again for another 8-10 minutes, or until no more white fumes are observed. Using your crucible tongs, carefully move the hot evaporating dish to a wire gauze or a clay triangle bent into the shape of a tripod, and allow the dish and its contents to cool in the hood. Do NOT put the hot dish down directly on the floor of the hood or the lab bench, because when the dish cools off, it may stick. 4. After the dish has cooled to room temperature, weigh the dish, salt and sand. C. Separation the Salt from the Mixture. 5. Add 25 ml of warm water to your sand/salt mixture to fill the evaporating dish, and stir several minutes with a glass stirring rod. Let the mixture settle, then decant (pour off) the salt water solution into a second evaporating dish which you have previously weighed together with a clean dry watch glass. Be careful to avoid pouring the sand out of the evaporating dish. Add another 10 ml portion of warm water, stir, allow it to settle, and decant the liquid. Repeat this process with still another 10 ml portion of warm water. This process extracts the soluble NaCl from the sand. You now have two evaporating dishes: one with an NaCl solution and the other with wet sand. 6. Carefully place the evaporating dish containing the NaCl solution on a hot plate (a clay triangle and a Bunsen burner may also be used). Heat gently to evaporate the water, taking care to avoid spattering. Near the end of the drying process, cover the evaporating dish with the watch glass that was weighed with the evaporating dish. Reduce heat to avoid spattering. While the water is evaporating, you may dry the sand in the other evaporating dish as explained in the next section. 9. When the NaCl is completely dry, no more water will condense on the watch glass. Let the evaporating dish and the watch glass cool to room temperature and weigh them. D. Drying and Weighing the Sand. 10. Position the evaporating dish containing the wet sand about 10-15 cm above the tip of the Bunsen burner flame to avoid loss of material by spattering. Cover the evaporating dish with a clean, dry watch glass. Heat the sand in the evaporating dish gently to evaporate the water remaining in the sand. As the sand approaches dryness, gently shake the dish to cause the sand to loosen and slide around, then heat strongly for five minutes. 11. Clean off the desk with wet paper towel at the end of the lab period. E. Percent recovery. 12. Determine that mass of each component recovered. You are expected to recover within 1% of the total mass and that the percentages of each component will be within 2-3%. If you have recovered less than 99% of the original sample, repeat the experiment with a new sample. If your results are within 1%, see your instructor to ensure that your determinations of individual components are sufficiently accurate. SEPARATION OF A MIXTURE: DATA TABLE (COPY INTO YOUR LAB NOTEBOOK) A. Original Sample Unknown Number Mass of evaporating dish Mass of evaporating dish and sample Mass of original sample (calculated) B. Mass of evaporating dish after sublimation of NH4Cl Mass of NH4Cl (calculated) Percent NH4Cl (show calculations) Calculations: C. Mass of evaporating dish #2 and watch glass (initial) Mass of evaporating dish #2, watch glass and NaCl Mass NaCl Percent NaCl (show calculations) Calculations: D. Mass of evaporating dish Mass of evaporating dish and SiO2 Mass SiO2 Percent SiO2 (show calculations) Calculations: E. Mass of original Sample Mass recovered (NH4Cl + NaCl + SiO2) Percent recovered (show calculation) Percent SiO2 (show calculations) POST LAB QUESTIONS: 1. Explain how your results might have differed if you had only completed one water wash of the sand/salt mixture instead of three? 2. Could this experiment been done in a different order (for example, if the mixture were first extracted with water could it have been quantitatively completed)? Explain. 3. A student found that his mixture was 13% NH4Cl, 18% NaCl and 75% SiO2. Assuming that the calculations are correct, what did h most likely do incorrectly?