The question is: A student performs a crystallization on an impure sample of biphenyl. The sample weighs 0.5g and contains about 5% impurity. Using his knowledge of solubility, he decides to use benzene as a solvent. The final weight is only 0.02g. Why is recovery so low?
My answer: The recovery is low because there will be some experimental loss, the original sample was not 100% biphenyl, and some biphenyl is soluble in the solvent even at 0 degrees C.
Is this sufficient?
Thanks from Sheryl
I would hit it little harder in spots. With a known impurity of 5%, we might expect to recover about 0.45 g. Some additional loss could be expected due to solubility in the solvent regardless of the solvent used. However, a recovery of only 0.02 g far exceeds looses due to solubility if the proper solvent was used for recrystallization and in the right amount. The loss in this case is due to the high solubility in benzene. Perhaps too much solvent was used. (The Merck Index says biphenyl is insoluble in water--which we would expect--and soluble in alcohol and ether. It doesn't mention solubility in benzene' however, solubility in ether suggests solubility in benzene,too My guess, then is that the student simply used too much benzene and his/her problem is loss in the solvent.)
Ha I'm doing the same problem right now. I'm pretty sure its because the biphenyl will dissolve completely in the benzene at room temperature (not good for crystallizations).
Thank you so much. I'm struggling for this question from last night.
Your answer is mostly correct. The low recovery can be attributed to several factors:
1. Experimental loss: There could be some loss of the sample during the process, such as during filtration or transfer of the crystals.
2. Impurities: The original sample contained about 5% impurity, which means that the recovered crystals will still have some impurities present.
3. Solubility in the solvent: Even if the proper solvent, in this case benzene, is used for recrystallization, there can still be some solubility of biphenyl in the solvent at a low temperature. This can result in loss of the desired crystals during the process.
4. Amount of solvent used: If too much solvent is used, it can lead to greater solubility of biphenyl, resulting in lower recovery of the desired crystals.
To improve the recovery, it is important to carefully control the amount of solvent used, ensure proper filtration techniques, and consider using additional purification methods if necessary.
Your explanation is mostly correct. Here's a breakdown of the reasons for the low recovery and some additional information:
1. Experimental loss: During the process of crystallization, it is not uncommon to experience some experimental loss, such as spillage or transfer errors. This could contribute to the low recovery.
2. Impurities in the original sample: The impure sample of biphenyl weighed 0.5g and contained about 5% impurity. Since the impurities are not biphenyl, their presence will reduce the overall weight of biphenyl that can be recovered.
3. Solubility in the chosen solvent: The student decided to use benzene as a solvent for the crystallization process. Benzene has relatively high solubility for biphenyl, even at low temperatures. This means that some amount of biphenyl would remain in solution and not be recovered during the crystallization process.
4. Excessive use of solvent: It is possible that the student used an excessive amount of benzene solvent, which could result in a larger volume of solution. This could further dilute the biphenyl concentration, making it harder to recover the full amount of biphenyl.
Considering these factors, it is likely that the low recovery is mainly attributed to the high solubility of biphenyl in benzene, and the possibility of using too much solvent. To increase the recovery, the student could try reducing the amount of solvent used or use a different solvent with lower solubility for biphenyl. Additionally, improving experimental techniques to minimize transfer errors could also help enhance the recovery.