E1W3 Prelab Ans

CH 241F EXPERIMENT #1, WEEK 3 PRELAB 1. You have 10 grams of a compound. Eight grams is soluble in 100 mL of boiling sol...

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CH 241F EXPERIMENT #1, WEEK 3 PRELAB 1. You have 10 grams of a compound. Eight grams is soluble in 100 mL of boiling solvent and 3 grams is soluble in 100 mL of cold solvent. What is the minimum amount of solvent you will need to dissolve your compound? What is the maximum yield of crystals you would expect? Show your calculations. The minimum amount of boiling solvent needed: 10 g X 100 mL/8 g = 125 mL The maximum yield of crystals: 3 g/100 mL X 125 mL = 3.75g left in solution! 10 g – 3.75 g = 6.25 g crystals (6 g is OK if you follow sig fig rules) 2. Provide a detailed procedure for testing solvents to determine if they are appropriate for crystallization of your compounds. Use 20 mg of compound and one mL of solvent in small test tubes. Explain exactly what you would do. Thoroughly understanding this procedure should save you a lot of time in lab. You are looking for a solvent in which your compound is quite soluble when the solvent is hot, but only very sparingly soluble when the solvent is at room temperature or lower. Procedure: Add 20 mg of your compound to 1 mL of solvent. Mix and observe. If the compound is soluble, add 20 mg more and mix again. If still soluble, the solvent is inappropriate for crystallization. Add 20 mg of your compound to 1 mL of solvent. Mix and observe. If the compound appears to be insoluble, heat the contents in a warm water bath, mix again and observe. If the compound does not dissolve appreciably in the hot solvent, that solvent is also inappropriate for crystallization. If the compound does dissolve in the hot solvent, it is necessary to cool the test tube in an ice-water bath to make sure that the compound is recoverable. 3. List two solvents that might be appropriate for the crystallization of fluorene and two for fluorenone. On what did you base your choices? Since fluorene is quite nonpolar, a polar solvent is the best bet for crystallization. Since fluorenone is somewhat polar, nonpolar solvents are likely to be good choices for crystallization. Fluorene: Ethanol is a good choice. Fluorenone: Ligroin or hexane(s) is a good choice.