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Rinse out the 250 mL flasks that you used to dilute the concentrated acid and base solutions. Weigh out 0.7 to 0.9 g of KHP (pictured below) into a 250 mL flask and dissolve it in 50 mL of distilled ...

Rinse out the 250 mL flasks that you used to dilute the concentrated acid and base solutions. Weigh out 0.7 to 0.9 g of KHP (pictured below) into a 250 mL flask and dissolve it in 50 mL of distilled water. Add three drops of the phenolphthalein indicator in the flask (see below). Then place the flask under the base buret and start the titration

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    Kh pacidbaseneutralization[1]slide onenewthis one Kh pacidbaseneutralization[1]slide onenewthis one Document Transcript

    • Crystallization and Purification Melting Point and Boiling Point of napthalene Ogranic Chemistry Dr. Robert D. Craig, Ph.D Pre Lab 1 1. A solid in which the atoms or molecules are arranged periodically. Within a crystal, many identical parallelepiped unit cells, each containing a group of atoms, are packed together to fill all space (see illustration). In scientific nomenclature, the term crystal is usually short for single crystal, a single periodic arrangement of atoms. Most gems are single crystals. However, many materials are polycrystalline, consisting of many small grains, each of which is a single crystal. For example, most metals are polycrystalline. 2. 7 step process of crystallization Step 1 Solvent Selection For an unknown solid, the solvent is selected by experimentation. The solvent might be one or two liquids (a solvent pair). An ideal solvent is one that has a steep solubility vs temperature curve with respect to the solid. That is, the solid is very soluble in the solvent when the solution is hot (boiling), but the solid is very insoluble in the solvent when the solution is cool (room temperature). You will select
    • one of three solvents, water, 95% ethanol, or the solvent pair t-butyl methyl ether/petroleum ether. Figure 1 shows structures of ethanol, t-butyl methyl ether and pentane. CH3 CH3CH2OH ethanol CH3 C O CH3 CH3CH2CH2CH2CH3 CH3 t-butyl methyl ether pentane Note: Petroleum ether is a mixture of hydrocarbons that have the properties of pentane. Step 2 Dissolution of the Compound The solid to be crystallized is placed in a test tube. The solvent (the better solvent of the pair if two solvents) is added to the test tube one drop at a time. A wooden splint or boiling chip is placed in the test tube to keep the solvent from overheating (becoming superheated). Air trapped within the wooden splint or boiling chip bubbles to the top of the liquid, causing the solution to boil evenly instead of all at once. When a superheated solution boils, it frequently results in a violent action called bumping; some of the solution might be lost. The boiling stick keeps the solution from bumping. Boiling chips work in a like manner, releasing air from within the pores of the porous chip. A boiling chip is more difficult to remove than is a stick, so a boiling stick is preferable. The solvent will be added to the test tube while the test tube is being heated on a sand bath. A sand bath (metal heating mantle filled with sand) is always used with a rheostat to control the temperature. A sand bath is never plugged directly into an electrical outlet. Enough solvent will be added to the solid to dissolve the solid, whereas, the insoluble material, charcoal in this experiment, will not dissolve. Step 3 Decolorization This step is sometimes necessary and sometimes unnecessary. It is necessary when a colored impurity must be removed from the solution. A small amount of either powdered charcoal or charcoal pellets is added to the solution. Colored impurities adhere to the charcoal and are removed with the charcoal when the solution is filtered. Charcoal pellets are preferable to powdered charcoal because pelletized carbon is easier to remove. Charcoal is used because it presents a large surface area on which the impurity can adhere. If there are no colored impurities to remove, this step can be eliminated. In our experiment, the charcoal has already been added. In this case, charcoal serves as both an impurity and as an adsorbent for any colored impurities that might be present. Step 4 Filtration with Fluted Filter (Removal of Impurities) You will use gravity filtration through a fluted filter paper to collect the unwanted solid impurities, including the charcoal. A piece of very porous filter paper is fluted by folding it in half, then in quarters,
    • eighths, etc. The fluted filter paper is then unfolded and placed into a stemless conical funnel. The funnel is stemless so that the solid does not crystallize in the stem during filtration. Your instructor will demonstrate how to flute a filter paper. The filter paper must be porous; otherwise, the solid crystallizes in the filter paper. Be sure to use the correct filter paper for this step. The fluted filter paper provides more surface area for the solution, making it more efficient than an unfluted filter paper. It is important for this step to be efficient to keep any solid from crystallizing in the filter paper. You will collect the filtrate in an Erlenmeyer flask. Step 5 Crystallization When a liquid (solvent) holds the maximum amount of a solid (solute) in solution at a given temperature, the solution is called a saturated solution. The fundamental rule of crystallization is that a solvent can generally hold more solid in solution at its boiling point than it can at room temperature. For example, if a given volume of solvent can hold 6 g of solid in solution at its (the liquid’s) boiling point and only 0.5 g of solid in solution at room temperature, then 6.0 – 0.5 = 5.5 g of solid can be recovered by crystallization from this volume of solvent. Thus, crystallization depends on the solubility of the solid at two temperatures, the boiling temperature and room temperature. The ideal solvent is one in which the solid is very soluble at the boiling temperature and very insoluble at room temperature. With this situation, the maximum amount of solid is recoverable. Thus, crystallization should start with a “hot” saturated solution near the boiling temperature of the solution. If too much solvent has been added, the solution will not be saturated. In which case, some solvent can be evaporated by heating the solution on the sand bath until the solution is saturated. The hot solution is then allowed to cool to room temperature. Because the “cold” solution cannot hold as much solid as the “hot” solution can hold, the solid will crystallize. This step is accomplished in the Erlenmeyer flask from the previous step. Hence, you will have a solid and liquid in the Erlenmeyer flask after the crystallization is complete. How do you know the crystallization is complete? You will observe the formation of crystals in your Erlenmeyer flask. When it appears to you that no more solid is forming, wait one or two minutes more, then cool the Erlenmeyer flask in an ice bath made by filling a beaker with ice and adding water so that the Erlenmeyer flask will be surrounded by ice water when it is immersed in the ice bath. Consider the example from above. If the solvent holds 0.2 g solid at the ice-bath temperature, then an additional 0.5 – 0.2 = 0.3 g of solid can be recovered, and only 0.2 g of solid will not be recovered from the crystallization. Step 6 Filtration with Suction (Collection of the Solid) The Erlenmeyer contains the desired solid and the solvent with a small amount of the desired solid still dissolved in the solvent. The liquid in contact with the solid is called the mother liquor and the solid— liquid mixture is called a slurry. This slurry is poured through a filter, which catches the solid and allows the mother liquor to pass through. If the desired solid contained a small amount of impurity, which is soluble in the solvent, that impurity will remain in the mother liquor. You will use either a Hirsh or
    • Büchner funnel to collect the solid. If you have less than 0.5 g of solid, you will use the Hirsh funnel. If you have a half-gram or more, you will use the Büchner funnel. The instructor will explain how to set up and use these funnels. The solid is then washed. Washing a solid is a technique in which a small amount of cold liquid is poured over the solid, while the solid remains on the filter. Washing results in the loss of solid, so only very small amounts of cold liquid should be used in the washing process. As much liquid as possible should be squeezed out of the solid. This is accomplished by pressing down on the solid with a piece of clean filter paper. When as much liquid as possible has been removed from the solid, the solid is ready for the next step. Step 7 Drying the Solid The filtered and washed solid is placed in a safe place to dry. The solid is transferred to a watch glass with a metal spatula. The watch glass is labeled and set aside until the next lab period. During this time, any solvent that remains adhering to the desired solid will evaporate, leaving a pure, dry solid. 3. Why are melting points recorded • Verifies the purity of a compound • Aids in the identification of an unknown • Typically reported as a range • An impure solid will have a lower melting point than the pure solid - less attractive forces within the solid, thus less energy to break up those forces, thus lower mp. Introduction Crystallization is a technique in which chemists use to purify solid compounds. It is one of the fundamental procedures each chemist must master to become proficient in the laboratory. Crystallization is based on the principles of solubility: compounds (solutes) tend to be more soluble in hot liquids (solvents) than they are in cold liquids. If a saturated hot solution is allowed to cool, the solute is no longer soluble in the solvent and forms crystals of pure compound. Impurities are excluded from the growing crystals and the pure solid crystals can be separated from the dissolved impurities by filtration. An organic compound’s melting point is one of several physical properties by which it is identified. A physical property is a property that is intrinsic to a compound when it is pure. Since melting points are relatively easy to determine, it is a handy identification tool in order to determine organic compounds in their pure and impure state.
    • Procedure A solution containing 2.0g of impure Naphthalene and 25mL of Methanol was prepared in a 50-mL Erlenmeyer flask. The mixture was heated until it came to a boil and then filtered through fluted filter paper into a 50-mL Erlenmeyer flask. 3mL of Methanol were used to wash out the original flask and the filter paper. The solution was heated to the boiling point and was allowed to evaporate until the volume was read to about 15mL. The heat was then turned off and distilled water was added drop by drop with the use of a pipette. After the addition of water the solution turned cloudy, the contents were swirled and any precipitated naphthalene was redissolved. After the addition of about 3.5mL of water the solution became close to saturated, the flask was removed from the heat and allowed to cool to room temperature. The crystals that formed were collected through the use of a Büchner funnel and suction. The crystals were then let out to dry and then weighed. The above procedure was repeated with 2.0g of an unknown instead of 2.0g of impure Naphthalene. The melting temperature range of the unknown was found experimentally and the unknown was identified. Structures of Compounds Observation and Results Crystallization of Naphthalene from Methanol Starting Mass of Impure Naphthalene = 2.0 Mass of Pure Naphthalene Retrieved = 0.615g Percent Yield = (Actual Yield ÷ Theoretical yield) x 100 = (0.615 ÷ 2.0) x 100 = 30.75% Crystallization of Unknown #3 from Acetone Starting Mass of unknown = 2.0g Mass of Pure unknown Retrieved = 0.13g Percent Yield = (Actual Yield ÷ Theoretical yield) x 100 = (0.13÷ 2.0) x 100 = 6.5% Meting Point Ranges Pure Naphthalene =80°C-81°C Pure Urea =134°C-138°C Pure Cinnemic acid =134°C-136°C
    • Unknown #3 =125°C-130°C Ratio(Urea-Cinnemic) Range(temperature) 1:4 100°C -125°C 1:1 100°C -115°C 4:1 98°C -115°C Identification of unknown The purified sample of Naphthalene, compared to the impurity that the experiment began with, was colorless. The impurities had been removed through the crystallization process. Not only was it evident that the impurities had been removed due to the color of the resultant Naphthalene, but it could also be seen in the difference between the starting mass and the mass retrieved. The percent yield was not 100%, which proved that something from the original sample had been removed. The same explanations hold true for unknown #3. The crystals that were obtained from the second crystallization process were weighed and their melting temperature range was measured and the unknown was therefore identified as Urea Data, Observations, Discussion and Conclusion In this experiment, we purified the naphthalene and got a white substance. After dried, we weighed it got 0.615g of naphthalene. The percent yield came up to be 30.75%. This percentage, show hoe distant to the theoretical yield we are. It was due to the fact that most of the product evaporated in the process of boiling. For the unknown, we chose the unknown #3 and tried with a small amount of unknown in each of the solvent available to find the one that does not dissolve it at room temperature. We found that Acetone did not dissolve the unknown so we used Acetone as the ideal solvent. We boiled the solution and the solute dissolved. After following the steps we got a precipitate and we used vacuum filtration to separate the precipitate. The melting point of unknown happened to be in the range of 125°C-130°C. The standard melting point of Cinnemic acid and Urea are the same; 132.5°C-133°C. But we received the range of 134°C -136°C for Cinnemic acid and 134°C-138°C for Urea, which are slightly above the standard range. We also received the ranges of melting points of the mixtures of Urea and Cinnemic acid which are; 100°C -125°C for the 1:4 ratio where ¼ of urea is added so it lowered the melting point of the pure Cinnemic acid and the
    • range is broader, which shows the contamination of the Cinnemic acid by urea. In the 1:1 ratio one, the range is between 100°C -115°C which is also lower than pure Cinnemic acid and also has a broader range which shows the impurity of the substance. In the 4:1 ratio, the range is 98°C -115°C which has a much broader range than the previous ratios because now the Cinnemic acid is contaminated with 4 times more Urea, in the mixture. During this experiment we observed that purification and crystallization method is to purify an impure substance by choosing an appropriate solvent to obtain the correct result with a maximum of percent yield. The solvent have to only dissolve the substance , when heated and leave other impurities as solid so it can be separated and then re-crystallize the solvent. Choosing the “ideal” solvent is the most important step to get the highest percent yield out of the final product. A hint given by the professor to create a “seed crystal” in order to begin crystallization was to add some of the ideal solvent to the solvent. Each substance has their melting point, and the melting point experiment can be used to identify the substance of the unknown. Also will be helpful to determine the impurity of the substance. The narrower the melting point range, the more pure is the substance. On the other hand, the broader the melting point range, the more impure is the substance. We also observed that an impure substance has a lower melting point than a pure substance. Post Lab questions 1. What is role of charcoal/activated carbon in the process of crystallization? The charcoal/activated carbon has a high surface area that has the capability to attract colored molecules and help to precipitate them out of the solution and decolorize it. 2. What is the advantage of using stem-less funnel over long-stem funnel while filtering hot solution through fluted filter paper? Using a stem-less funnel doesn’t give the solution a chance to cool and clog the stem with crystals .The fluted filter paper has a larger surface area than a regular un-fluted filter paper which allows a rapid rate filtration. 3. When measuring the melting point of a sample how one should heat the melting point bath/block and why? The rate of heating should not be faster than 1°C per minute. The melting point range begins when the first crystal begins to melt, if you begin from a high
    • temperature and the sample begins to melt down fast and it would be impossible to observe the first crystal melting. Therefore range of melting point would be determined inaccurately. 4. How can you differentiate a pure and an impure sample by measuring their melting points? The narrower the melting point range, the more pure is the sample, and the broader the melting point range, the more impure is the ample .Also an impure sample has a lower melting point than a pure sample. 5. For the re-crystallization of a sample one should choose a solvent on the basis of what property? The property of “polarity” of the solvent is used to choose the “ideal” solvent.