Recrystallization Post Lab Questions 1 8pts Consider a heterogeneous mixture of two unknown solids Q S Determine which of the following solvents dimethylsulfoxide ethyl acetate or dichloromethane would an appropriate choice or choices for isolating and recrystallizing Unknown Q from Unknown S Explain the reasoning behind your decision s in detail In addition explain why the remaining solvent or solvents would be not be acceptable options for the proposed experiment Solubility of Unknown Q Solubility of Unknown S g 5mL g 5mL Dimethylsulfoxide 22oC 85oC Ethyl Acetate 22oC 85oC Dichloromethane 22oC 85oC 0 005 0 047 0 975 0 751 0 623 2 91 0 157 7 14 2 51 2 75 0 712 3 43 The obvious best choice is DimethylSulfoxide since Unknown S is highly soluble while unknown Q is not very soluble thus making it easier to separate from one another 2 4pts If you allowed your solution of hot water impurities and solute to cool on the benchtop before employing a room temperature gravity filtration what is the most likely outcome for your separation Will it result in a successful separation of the solute and impurities What is likely to happen with your solution under these conditions Explain your reasoning in detail If we were to let the solution cool on the benchtop before running it through the gravity filtration it would have the crystals form on the filter paper thus meaning that the filtration was ultimately unsuccessful as the crystals would form along side the other impurities As for the solution there would probably be very few crystals as most of them would have formed alongside the impurities that are on the filter paper 3 3pts After recrystallizing and isolating an unknown crystalline solid you obtain a preliminary melting point range of 132 137oC Identify your purified unknown using the table below Which of these compounds is your unknown most likely to be How did you arrive at your decision Explain your reasoning in detail Compound Name Melting Point oC Benzoic Acid Benzamide Cinnamic Acid o Nitrobenzoic Acid Salicyclic Acid 121 123 128 130 133 135 146 148 158 160 I would think that the purified unknown compound is cinnamic acid because the range is closest to cinnamic acid considering the maximum range is closest to that of the cinnamic acid rather than o Nitrobenzoic acid The slight differences in the melting points is most likely a result of the impurities that are still left behind considering this was a preliminary melting point range that was given 4 4pts A crystalline compound K is known to be soluble in dimethylformamide DMF at 41oC It is possible to dissolve a total of 20 12 g of solid K in 150 mL of DMF If you are given a 25 4 g sample of this unknown compound assuming that any impurities present are insoluble in dimethylformamide at room temperature what is the precise volume of DMF you would require to perform a full recrystallization of solid K In order to receive full credit you must show your calculations and explain your reasoning 150 mL 20 12 g 25 4 g 189 364 mL We would require 189 364 mL of DMF in order to perform the recrystallization of K We can ultimately accomplish recrystallizing K by mixing the 25 4 g of our unknown compound and the necessary 189 364 mL of DMF heating the dissolved mixture of the two letting it cool to room temperature and then lastly separating it via filtration The reason this works is because the impurities are insoluble at room temperature so when you use filtration the regular solution will pass through the filter allowing for the crystallization of solid K while the impurities will be unable to pass through the filter paper 5 3pts After recording a preliminary melting point range of 140 147oC for an unknown solid you identify it as o nitrobenzoic acid Running a mixed melting point determination with o nitrobenzoic acid affords a melting point range of 126 135oC Why did your melting point change so dramatically What does this tell you about your initial assumption concerning the identity of your unknown solid The initial assumption concerning the identity of the unknown solid isn t wrong instead the difference in melting point ranges is more than likely the result of impurities that still remain within the sample 6 2pts Would it be a good or bad idea to use a large excess amount of water to dissolve your unknown when recrystallizing it What are the ramifications of this action It s a bad idea to have large amounts of excess water used to dissolve the unknown because adding too much will result a decreased percent recovery
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