EXPERIMENT #3: SEPARATION OF A MIXTURE OF SOLIDS Purpose: The purpose of this experiment is to learn methods of separating a mixture of substances and to perform calculations involving mass percentages. Special Apparatus and Chemicals: Bunsen burner 2 Evaporating dishes Iron ring and stand Electric hot plate Tongs Watch glass Clay triangle 2-3g of unknown mixture Discussion: Since chemistry is the scientific study of the composition, structure, and properties of matter and the transformations that it undergoes, chemists are often called upon to determine the composition of a material. If the material is a mixture, the components of the mixture may have to first be separated in order to quantitatively determine the composition. Materials may be classified as either pure substances or a mixtures. Pure substances are those materials defined by a fixed composition and a set of characteristic properties. A pure substance may be either an element or a compound. Mixtures are combinations of two or more pure substances in a variable ratio. Mixtures may be classified as heterogenous or homogenous. Homogenous mixtures (often called solutions) have a uniform composition throughout; heterogenous mixtures have a variable composition in different regions of the material. Mixtures may be separated into pure substances based on differences in the physical properties of its component pure substances. Physical properties are those properties that can be measured without changing the basic nature of the substance. Some examples of physical properties are melting and boiling points, color, odor, density, solubility, and hardness. In this experiment we will use differences in the properties of sublimation and solubility to separate a heterogenous mixture of solid NH4C1, NaC1, and Si02. Because ammonium chloride (NH4Cl) undergoes sublimation at 340°C and NaC1 and Si02 do not, the NH4C1 may therefore be separated from a mixture containing NaCl and Si02 by heating the mixture containing all three above 340°C. A substance is said to undergo sublimation when it goes from the solid phase directly to the gas phase without passing through the liquid state. Some common substances that undergo sublimation under normal conditions are CO2 (dry ice), iodine, naphthalene (moth balls), and ice (below the freezing point, 0°C). Upon heating the mixture containing NH4Cl, the ammonium chloride will be seen leaving the sample as a white smoke. As soon as the ammonium chloride leaves the dish, the sublimed NH4C1 condenses back to a solid which is seen as white smoke. Both NH4C1 and NaCl are soluble in water but SiO2 is not. NH4C1 and/or NaC1 could be extracted from a mixture with SiO2 by adding water to preferentially dissolve the NH4C1 and/or NaC1. The aqueous solution of NH4C1 and/or NaC1 that forms could be removed from the still solid SiO2 by decantation or filtration. Decantation is the process of carefully pouring a liquid from a solid. Filtration is the process of using a porous membrane (i.e. filter paper), allowing a liquid to pass through leaving the solid behind on the filter. Once you have determined the mass of the components in a mixture, and the total mass of the sample is known, the percent by mass of the components in the mixture may be determined. The composition of mixtures is often reported as a mass percent. "Percent" means per centuin the amount out of 100. Percents are calculated by taking a fraction or ratio and multiplying by 100. To calculate a mass percent take the mass fraction and multiply by 100. For example if a mixture contains two components A and B with a total sample mass of 2.454g and there is 1 .002g of A in the mixture then the mass percent of A in the mixture may be calculated as follows: Concept: The unknown mixture that you will separate contains 3 components, NaCl (sodium chloride, "table salt"), SiO2, (silicon dioxide, "sand" or "silica") and NH4Cl (ammonium chloride). The separation of these three compounds will be accomplished by first heating the mixture to remove NH4Cl by sublimation. Next, extracting the NaCl from the mixture with water, and finally removing the remaining water from the NaCl and SiO2. See scheme above: Procedure: Weigh a clean, dry evaporating dish. Add between 2 to 3 grams of your unknown mixture and weigh the dish and the sample and determine the sample mass to the full limit of precision of your balance. Put the evaporating dish with the mixture on a ring stand IN THE FUME HOOD and heat the mixture over a Bunsen burner until white smoke stops forming. CAUTION: Sublimation must be done in the hood because a large amount of NH 4Cl smoke is produced. Be careful to make sure that all of the NH 4Cl has been removed from the dish, check to make sure none has recondensed on the side of the dish. Crucible tongs may be used to move the dish and a wire gauze pad should be used as a hot pad. Cool the evaporating dish to room temperature and weigh the dish and sample after heating. (NEVER WEIGH HOT OBJECTS). The mass difference before and after heating is the amount of NH4Cl sublimed. Calculate the mass percent of NH4Cl in the original sample. Alternative Heating Method. If Bunsen burners are unavailable, the sublimation step may be carried out by placing the sample in a evaporating dish or Petri dish bottom and heating on an electric hot plate turned on high. Remember to do this step in the HOOD. Add 20 mL of distilled water to the solid remaining in the evaporating dish and stir to dissolve the NaCl. Weigh the second evaporating dish and watch glass empty. After letting any SiO2 present settle to the bottom, decant (pour off) the liquid from the first dish to the second. Be careful not to transfer any of the SiO2, sediment to the second dish. Add 10 more mL of water to the first dish, stir, and then again decant liquid to the second dish. Repeat with a second 10 mL portion of water. Now the first dish contains wet SiO2 and the second an aqueous solution of NaCl. Heat both dishes carefully on a hot plate until all of the water evaporates. When the dish containing NaCl is near to dryness, cover with the watch glass to prevent splattering as the last of the water evaporates. After all the water has evaporated from both dishes (make sure the watch glass is completely dry also), cool to room temperature, and weigh both dishes. Calculate the amount of NaCl and SiO2, that you have recovered from the sample. Calculate the mass percentage of NaCl and SiO 2 that were in the original sample. DISPOSAL: The NaCl left in the evaporating dish may be washed down the sink since it is non-toxic and water soluble. The leftover SiO2 should be dumped in the trash can. Add the masss of NH4Cl, NaCl, and SiO2 that you determined to be in your sample. Calculate the percent of matter that you recovered. If your combined total is not between 99.0% and 100.0%, explain what mistakes may have occurred in your experiment. Some common mistakes include not completely drying the NaCl or SiO2, letting some of the NaCl solution splash out of the dish during evaporation, and material popping out of the dish during sublimation or evaporation. Consult your instructor to find out if you should repeat the procedure. Name: _______________ Section: ______________ Date: ________________ Unknown number: ___________________ Mass of Original Sample Mass of dish #1 and original sample _____ _____ _____ g Mass of dish #1 _____ _____ _____ g Mass of original sample _____ _____ _____ g Separation of NH4C1 from Mixture Mass of dish #1 and original sample Mass of dish #1 after subliming NH4C1 Mass of NH4CI removed Percent of NH4C1 in original mixture Average percent NH4C1 _____ _____ _____ g _____ _____ _____ g _____ _____ _____ g _____ _____ _____ % _____% Separation of SiO2 from Mixture Mass of dish #1 and SiO2 Mass of evaporating dish #1 Mass of SiO2, Percent of SiO2 in original mixture Average percent SiO2 _____ _____ _____ g _____ _____ _____ g _____ _____ _____ g _____ _____ _____ % _____% REPORT SHEET: SEPARATION OF A SOLID MIXTURE, continued Separation of NaC1 from Mixture Mass of dish #2, watch glass, and NaC1 Mass of dish #2 and watch glass Mass of NaC1 Percent of NaC1 in original mixture Average percent NaC1 _____ _____ _____g _____ _____ _____g _____ _____ _____g _____ _____ _____% _____ _____ _____% Recovery of Components Mass of original sample Sum of masses of individual components determined Percent of matter determined Average percent determined _____ _____ _____g _____ _____ _____g _____ _____ _____% _____ _____ _____% If the percent of matter determined is less than 99.0% or greater than 100.0%, explain what errors occurred in your experiment. _____________________________________________________________________ ________ ____________________________________________________________________ ___________________________________________________________ ____________________________________________________________ _____________________________________________________________________
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