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Organic solids are frequently purified by recrystallization - Leaving Cert Chemistry - Question 3 - 2011
Question 3
Organic solids are frequently purified by recrystallization. An impure sample of benzoic acid contained small quantities of the two solids: salt (NaCl, white and so... show full transcript
Worked Solution & Example Answer:Organic solids are frequently purified by recrystallization - Leaving Cert Chemistry - Question 3 - 2011
Step 1
What precaution should have been taken at Stage 1 to ensure the maximum yield of pure benzoic acid crystals at Stage 5?
Answer
To maximize the yield of pure benzoic acid crystals, it is crucial to dissolve the benzoic acid in the minimum amount of hot solvent (water). This ensures that a saturated solution is obtained, promoting the formation of more crystals during the cooling process at Stage 3. Additionally, heating the solution until almost saturated can help prevent the premature crystallization of impurities.
Step 2
What solid was collected (i) at A, (ii) at B?
Answer
(i) At A, the solid collected is charcoal (C), which is the insoluble solid that does not dissolve in the hot solvent. (ii) At B, the solid collected is pure recrystallized benzoic acid, as it is soluble in hot water but precipitates out during cooling.
Step 3
Explain how the solubilities of benzoic acid and salt (NaCl) in hot and cold water, explain how benzoic acid is separated from the salt in this procedure.
Answer
Benzoic acid is much less soluble in cold water compared to hot water, while sodium chloride (NaCl) is more soluble in cold water. This key difference allows for the separation of benzoic acid from NaCl during recrystallization. When the solution is cooled, benzoic acid crystallizes out because it is less soluble at lower temperatures, leaving the NaCl in solution, as it remains soluble even as the solution cools.
Step 4
Describe how the benzoic acid was dried at Stage 5.
Answer
At Stage 5, the benzoic acid is dried by placing it in a warm area or in a desiccator. Alternatively, it can be left to air dry on a filter paper or a drying tray. This process helps remove any residual moisture without causing degradation of the benzoic acid.
Step 5
Draw a diagram of a labelled diagram how the melting points of the impure benzoic acid is determined, after recrystallized, discard and clean beakers.
Answer
To determine the melting point of the recrystallized benzoic acid, use a melting point apparatus. Attach a thermometer to the melting point tube containing the sample. Heat the sample gradually in a controlled manner, noting the temperature range at which the benzoic acid begins to melt and observing any changes in its state. Clean all used beakers thoroughly before repeating the process with the impure sample for comparison.
Step 6
How would you have expected the melting point values of the two samples to differ?
Answer
The melting point of the pure recrystallized benzoic acid would be expected to be sharp and close to its known pure value, typically around 122 °C. In contrast, the melting point of the impure benzoic acid would likely be lower and wider, indicating the presence of impurities that disrupt the orderly crystal structure.
Step 7