Given the following solubility test results, which would be the best recrystallization solvent for Compound A? Why? Ethanol: Cold – soluble; Hot – soluble Water:Cold – insoluble; Hot – soluble Benzene:Cold – insoluble; Hot – insoluble Water would be the best recrystallization solvent for Compound A, because when doing recrystallization, it is important that the compound be very soluble at the boiling point, and hardly soluble at room temperature. If it were insoluble in hot temperatures the compound would not dissolve, where as if it was soluble in cold, crystillization would be a very unlikely process.
To note, impurities should on the other hand be very soluble at room temperature and insoluble in hot solvent in order to be filtered out due to the fact they remain dissolved in the solvent where as the rest of the compound crystallizes when cooling. 2. How does recrystallization purify a solid? -First a saturated solution is made(a compound containing impurities is dissolved in a hot solvent) -Secondly, the solution is cooled slowly(the slower, the bigger the eventual crystals), allowing solubility of compound to drop(not impurities solubility however), starting the crystillization process.
(to note, to start the crystillization of the compound a seed is often required which involves scratching the glassware until a nucleus of glass provides a surface on which crystals may begin to form) -Ideally by this step, solid equilibrium of impurities is not exceeded so crystals have absolutely no impurities. (in this case we could just filtrate discarding filtrate with all impurities) -However this is near impossible to achieve, impurities(however small an amount) make it into crystals.
In this case we recrystallize(heat and add water as solvent, cool, filtrate) again and again until pure. -Melting point helps indicate the amount of impurities in the crystals. (impurities lower expected melting point) 3. Briefly explain the significance of the melting-point results in regards to the purity of the crude and recrystallized product. The melting point for 100% pure acetaminophen should have been between 169-172 degrees celcius.
A melting point range of 151-154 was observed prior to recrystallization(signaling significant amounts of impurities), and a range of 161-165 after recrystallization(showing an improvement, and small purification leading to a decently pure product). 4. Suggest one chemical reason, specific to this experiment, why your percent recovery could be less than 100%. Initial crystillization of the saturated solution accounts for loss of product(a small percentage of the pure compound will not crystallize because it is not 100% insoluble at room temperature), also charcoal may absorb a small amount of product, though mostly absorbing impurities.
In conclusion the more the compound is purified the less product is recovered. In this case the melting point and appearance points to quite a pure compound meaning to get to that desired state, much product would have been loosed in the process do to procedures. References; -http://danielleamorim. tripod. com/ -McGill CHEM 212 Recrystallization lab manual/mycourses
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