• A student crystallized a compound from benzene and observed only a few crystals when the solution cooled to room temperature. To increase the yield of crystals, the students chilled the mixture in an ice-water bath. The chilling greatly increased the quantity of solid material in the flask. Yet when the student vacuum filtered these crystals, only a few crystals remained on the filter paper. Explain what this student might have done wrong to obtain this outcome.
Based on the information given, it appears that the student might have made a mistake during the vacuum filtration step. Vacuum filtration is a method used to separate solid crystals from a liquid by applying a reduced pressure to draw the liquid through the filter paper while leaving the solid behind. Here are a few possible explanations for the low yield of crystals in this case:
1. Insufficient chilling: The student mentioned chilling the mixture in an ice-water bath to increase the yield of crystals. However, if the chilling period was not long enough or the temperature was not sufficiently low, it may not have facilitated the complete crystallization of the compound. As a result, only a few crystals would have formed, leading to a low yield.
2. Poor filtration technique: The student mentioned vacuum filtration as the method used to separate the crystals from the liquid mixture. It is possible that the filtration setup was not properly assembled or used. If the vacuum pressure was too high or the filter paper was not properly placed, some of the crystals might have been lost during the filtration process.
3. Incorrect choice of filter paper: Filter paper comes in various pore sizes, and the student may have chosen a filter paper with pores too large for the size of the crystals. In such a case, the crystals could pass through the filter paper along with the liquid, leading to a low yield.
4. Loss during transfer: It is also possible that the crystals were lost during the transfer from the flask to the filter paper. If the transfer was not done carefully or if some of the crystals stuck to the sides of the flask or other equipment, the yield would be decreased.
To improve the outcome and obtain a higher yield of crystals, the student should consider the following steps:
1. Ensure sufficient chilling: Allow the mixture to cool for an adequate amount of time at a low temperature to promote complete crystallization.
2. Optimize filtration technique: Properly assemble the vacuum filtration setup, ensuring that the filter paper is securely placed and the vacuum pressure is appropriate for the filtration. Consider using a filter paper with a smaller pore size if necessary.
3. Minimize crystal loss during transfer: Be careful when transferring the crystals from the flask to the filter paper to avoid any loss. Use appropriate tools (such as a spatula) to carefully scrape and transfer the crystals.
By taking these precautions, the student should be able to improve the yield of crystals obtained during the vacuum filtration process.