# Separating a Mixture, Recrystallization, pre-lab assignment could you also explain why you chose that substance for...

Separating a Mixture, Recrystallization, pre-lab assignment

could you also explain why you chose that substance for the empty spaces and question marks
EXPERIMENT 4 Pre-Lab Assignment Separating a Mixture, Recrystalliration Name Date 1. Complete the following flowchart which shows how to separate a mixture of sand, sodium chloride and acetanilide. Notice that after a separation process (a down arrow) the filtered solids are shown on the left and the filtrate (the liquid) is shown on the right. The terminal step should give a pure product. (? next to arrows indicate procedures: 7 below arrows are the product of a procedure.) Then, on a separate sheet of paper, type a 300 word summary, in your own words, that explains the rationale behind your flowchart: what gets separated at each step, and why water is a useful solvent for this re- crystallization Sand, salt, acetanilide Obtain mass to 0.001 Mix with hot water, then filter Filtered material Filtrate (liquid that passes through filter) Drycrystals Filtered material Drycrystals 2. What are the characteristics of a solvent that are useful for recrystallization?
3. Suppose your sample was 26,2% acetanilide and the sample mass was 5231 Considering acetanilide's cold water solubility of 0.0054 g/mL, what is the maximum mass of acetanilide you could collect if the sample was exposed to a total of 45 ml of cold water? 4. Give two examples of each of the following: a) A homogeneous mixture of b) A heterogeneous mixture two substances of two substances (eg: saltwater) (eg: oil mixed with water) c) A heterogeneous mixture of a pure substance (eg: ice floating in water) Snoilssilsiz
AD Separating a Mixture, Recrystallization OBJECTIVES 1. Separate the components of a mixture based on component solubilities in water. 2. Determine the percent composition of the mixture. 3. Learn lab techniques of filtration and recrystallization EXPERIMENT SUMMARY In this experiment, you will devise a scheme to separate three components of a mixture, relying on their differences in solubility in water. You will also determine a method for calculating the percent of each component in the mixture. A variety of unknown mixtures will be available for analysis, and you will be graded on both your pre-lab procedure and the results of your experimentation. DISCUSSION AND THEORY Most chemical reactions that occur do not give tidy, pure substances as products. They give mixtures. A typical reaction, for example, rarely goes to completion, so the products (if not gaseous or precipitated out of the reaction medium) are contaminated with starting reagents: a mixture. Many reactions give two or more products: a mixture. Some reagents can combine in more than one way to form yet another mixture. So one of the premiere tasks of experimental chemists is to separate the components of a mixture. A pharmaceutical chemist needs to isolate, in pure form, the new wonder drug he is synthesizing. An analytical chemist needs to separate the pesticide residue in the fruit before she can determine what it is, and how much of it there is. The petroleum chemist needs to separate the black sludge of crude oil into its useful components: candle wax. motor oil, gasoline, and so forth. A pure substance is one compound or element that is uncontaminated with any other compound or element. No matter what the origin of a pure substance, or how it is prepared, or what quantity is represented, it will always have the exact same elemental composition and the same physical and chemical properties. A French chemist. Joseph Proust, in 1797. recognized this in his Law of Definite Composition that different samples of a pure substance will contain the same elements in the same proportions by mass. For example, when pure water is analyzed it is always found to be exactly 11.2 % hydrogen and 88.8% oxygen, no matter if the water were pulled out of the Amazon River and purified, obtained from a rain barrel in Georgia, melted from snow in Idaho, or prepared in a Boston laboratory by exploding hydrogen with oxygen A mixture, on the other hand, is a physical, not chemical, combination of two or more une substances. Its composition is not fixed, but can vary, according to how the mixture
was prepared. For example, a mixture of sodium chloride in water might be 1.0 % salt and 99.0% water, or 20.0% salt and 0.05 water. In fact, the composition of this mixture can vary infinitely, depending on the relative amounts of water and salt, and each composition will have a set of properties unique to its a unique boiling point, melting point, index of refraction, density, ete. An important distinction between mixtures and pure substances is that mixtures, being prepared by physical methods, often may be separated into their constituent compounds by physical means. A pure substance that is a compound, however, can only be resolved to simpler species by chemical methods. (A pure substance that is an element, of course, cannot be broken down to any simpler substance.) The above mixture of salt and water can be separated by the physical process of distillation, where the water is boiled away from the salt and then condensed. Chemists have many techniques to resolve a mixture and separate it into the pure substances that make it up. 177007717722711 MATTER PURE SUBSTANCE 1. Only 1 compound or element is present 2. Definite and fixed composition 3. Constant properties under given conditions independent of quantity MIXTURE 1. Two or more pure substances mixed by physical means 2. Variable composition 3. Properties vary according to composition 4. Separate components by physical methods HETEROGENEOUS Visibly different components each with different properties ELEMENT 1. One of the ultimate 108 building blocks of all matter 2. Cannot be resolved to any simpler substance 3. Smallest units are atoms HOMOGENEOUS No visibly different components, Uniform throughout COMPOUND 1. Two or more elements chemically bound together 2. Definite and fixed elemental composition 3. Need chemical methods to separate into component elements 4. Smallest unit is either molecule or positive and negative ions
>>2X There are two types of mixtures, heterogeneous and homogeneous. If salt is dissolved in water, a solution is formed which visually looks exactly the same as pure water. Just by looks alone you could not distinguish between the two. Such a mixture, which is composed of one phase that is visually uniform throughout, is known as a homogeneous mixture. A heterogeneous mixture, on the other hand, is composed of visually distinctive parts, each with unique properties. Examples of heterogeneous mixtures are sand mixed with water, or powdered sugar mixed with salt, or chipped ice floating in water. In each of these there are two visually distinctive materials present. The last example represents a heterogeneous mixture of different phases of a single pure substance. The table below summarizes the classifications of matter. From the table we see how the properties of mixtures vary from the properties of pure substances, and now we can explore ways to separate mixtures into their pure components. Chemists have a variety of tools for accomplishing this. They may use distillation to separate liquid components that have a fair difference in boiling point for example. For solids they might exploit differences in the solubility of components, then use techniques of filtration. We'll focus on this type of purification in this experiment. For this lab your unknown will be a solid mixture of three substances: sand, acetanilide, (pronounced ass-et-an-ill-id) and salt (sodium chloride). Your first task, in the pre-lab assignment before you come to class, is to develop a procedure by which this mixture can be separated quantitatively into its components. You will need to know the following solubility information: *properties Compound In cold water In hot water sind insoluble insoluble acetanilide insoluble (0.0054 g/mL @ 20°C) soluble (6.8 g/mL @ 95°C) sodium chloride (salt) soluble soluble To achieve a workable plan, you will need to use simple liquid-solid separation techniques. If a solid is heavy and granular, you may decant the liquid away from the solid. A liquid can also be filtered away from a solid using filter paper. When all of the liquid is filtered, the solid can be collected by washing it out of the beaker onto filter paper. Before weighing the solid, it will have to be dried thoroughly. The easiest way to do this is to simply wait until the next lab period, when all of the liquid has evaporated away. If the solid is completely insoluble in acetone, then this volatile, water miscible liquid can be used to rinse the solid, and will easily evaporate away. Boiling the water away from a salt/water mixture is another option for isolating the sodium chloride. Caution: acetone fumes may be explosive. Do NOT use flame drying on any residue containing acetone. Recrystallization is a method of purifying a crystalline solid. A solvent is selected which dissolves a substantial amount of the solid when the solvent is hot, but very little when cold. An ideal solvent also evaporates easily i.e., has a low boiling point), is not toxic,
and does not react with the substances being purified. The hot solution is gravity filtered to separate the impurities that did not dissolve. When the filtered hot solution cools down, the solid will begin to crystalline out of the solution, usually in a purer form than it was originally. Use a vacuum filtration to separate the pure crystals from the mother liquid", which contains the soluble impurities Note that the two types of filtration mentioned above, gravity and vacuum are to be used for specific purposes. Use gravity filtration if the filtered liquid is hot, because the Vacuum method may cause flash boiling when the hot liquid enters the vacuum. Use Vacuum filtration to collect a solid from a cold liquid. The air flowing over the solid will help to dry it. Again, the solid must be completely dry if good results are to be obtained Your instructor will demonstrate these techniques, and lead a class discussion to arrive at a viable set of procedures for you to follow so you can achieve the best results. The illustrations below can show you the proper techniques for decanting, filtering, and vacuum filtration Stirring rod d Wash Bottle Buchner Funnel PILSSTIPI To vacuum Collecting a Precipitate Decanting off the Liquid Vacuum Filtration While you are making up your pre-lab plan, you must decide how to quantify the composition of your particular unknown mixture. Different unknown mixtures will be available to the class, each with a different composition. The percentage of any given component is given by the formula: % component weight pure component - 100 total weight of mixture You must devise a plan to determine the mass of each pure constituent in the unknown mixture, so that the percentage of each can be determined. The only constraint (other than using the Department's mass spectrometer) is to use only about 5 grams of the unknown. We also suggest that you cross check the mass of acetanilide indirectly, calculated from the mass of the sand, salt and the mass of the total mixture, to account for the loss of some acetanilide in cold filtration.
After a series of steps you will eventually end up with crade acetanilide. Base the mass of the acctanilide in the sample on this crude acetanilide after it has been dried, and determine its melting point. Recrystalline the acetanilide from a minimal amount of water and, after you allow it to dry, determine the melting point of this purified acetanilide. You may lose a lot of the acetanilide in this step (particularly if you are careless in the amount of water you use) so do not base the percent acetanilide in the sample on the mass of this purified acetanilide just determine if its melting point has improved from the crude material. After reporting your results to your instructor, you will be given the correct percentages of components of the mixture. Calculate the % error of your answer.

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