Assume three students have a two component mixture of two organic molecules (gen
ID: 888877 • Letter: A
Question
Assume three students have a two component mixture of two organic molecules (genetically called "oval" and "square') dissolved in a minimum amount of a boiling volatile solvent. The illustration below is meant to pictorially represent the relative quantities of each component. Thus, there is much more of oval and much less of square. Each student proceeds with the recrystallization experiment in a different way. For each procedure sketch what their experiment will look like. Assume oval and square have equivalent solubility profiles (see NOTE in #2 on previous page) in the solvent. Decisive Dan puts his boiling flask in a refrigerator to rapidly cool his solution. Luckily the specialty lab glassware doesn't crack as it is rapidly cooled. When Dan retrieves his solution an hour later sketch what it might look like with respect to the oval and square molecules and solvent. Assume the temperature of the refrigerator is near 0 degree C but the freezing point of the solvent is much lower than 0degree C. Always Allie decides to pour extra solvent into her flask (nearly doubling the volume) before letting her solution slowly come to room temperature in her hood. After several hours she is sad. Sketch what her flask looks like. What was the problem? What can she do to solve her problem? Briefly explain.Explanation / Answer
All students expect to have some recrystallisation of “ovals” after the solution is cooled, because the initial hot solution is saturated in “ovals”(a minimum volume of hot solvent was used) but not saturated in “squares” (the “ squares” have the same solubility but are in a much lower initial concentration).
a) Dan rapidly cooled the solution. He will obtain only very small and floating crystals (only a turbid/cloudy/unsettled solution). It not possible to proceed to a separation by decantation.
Draw on the picture a cloudy solution.
b) Allie doubled the initial volume of the solvent and the initial hot solution is not saturated. She has a very low chance to see a recrystallization after cooling.
Draw on the picture a clear solution (only the solvent line/level).
She needs to evaporate the added volume of the solvent, at boiling and to proceed further like Sophia.
c) Sophia has cooled the solution very slowly. The crystals can now grow. She will obtain a deposit of “ovals” crystals (it is possible that no ‘square” is recrystallised, their saturation at room temperature is not riched). She can separate the deposit by decantation.
But the solution still contain dissolved “ovals” and “squares” .
Draw on the picture: floating (dissolved) “ovals” and “squares” and a deposit of “ovals”.