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A sample of impure benzoic acid was recrystallised as follows: 2.5 g of the impure benzoic acid was weighed out and dissolved in the minimum amount of hot water - Leaving Cert Chemistry - Question 3 - 2004
Question 3
A sample of impure benzoic acid was recrystallised as follows: 2.5 g of the impure benzoic acid was weighed out and dissolved in the minimum amount of hot water. The... show full transcript
Worked Solution & Example Answer:A sample of impure benzoic acid was recrystallised as follows: 2.5 g of the impure benzoic acid was weighed out and dissolved in the minimum amount of hot water - Leaving Cert Chemistry - Question 3 - 2004
Step 1
Why is it important to use the minimum amount of water in the procedure?
Answer
Using the minimum amount of water is crucial because it maximizes the yield of the recrystallised benzoic acid. If too much water is used, the benzoic acid may remain dissolved, leading to a lower yield. Additionally, using a concentrated solution increases the likelihood of crystal formation upon cooling, as a saturated solution will promote crystallisation.
Step 2
Indicate clearly the stage of the recrystallisation procedure at which insoluble impurities were removed and how their removal was achieved. Indicate also the stage at which soluble impurities were removed and how their removal was achieved.
Answer
Insoluble impurities were removed during the initial filtration stage of the recrystallisation process. This was achieved by filtering the hot solution through a filter paper in a funnel, where insoluble materials remained on the filter paper while the clear solution passed through.
Soluble impurities were removed during cooling and recrystallisation. Upon cooling, only the purified benzoic acid forms crystals, while the soluble impurities remain dissolved in the mother liquor. These impurities are eliminated when the crystals are collected by filtration once the solution crystallises fully.
Step 3
How could you have ensured that the recrystallisation was complete?
Answer
To ensure that recrystallisation was complete, one could check the purity of the crystals by performing a melting point determination. Pure substances have a specific melting point range, and if the measured melting point is sharp and falls within the expected range for pure benzoic acid, it indicates that recrystallisation was successful. Additionally, visual inspection can be done to ensure that no more crystals are forming during the cooling stage.
Step 4
Step 5
What was the percentage yield of purified benzoic acid?
Answer
The percentage yield of purified benzoic acid can be calculated using the formula:
ext{Percentage Yield} = rac{ ext{Actual Yield}}{ ext{Theoretical Yield}} imes 100
In this case:
ext{Percentage Yield} = rac{2.25 ext{ g}}{2.5 ext{ g}} imes 100 = 90 ext{ \\%}
Step 6
Describe with the aid of a labelled diagram how you would have measured the melting point of one of these samples.
Answer
To measure the melting point accurately, you would set up a melting point apparatus that includes a small capillary tube containing the sample, and a thermometer. The thermometer, which measures the temperature of the melting block, should be placed close to the capillary tub. Gently heat the sample slowly, observing the temperature at which the solid starts to melt completely. A labelled diagram would show the melting point apparatus, the sample in the capillary tube, and the thermometer positioned for accurate temperature readings.
Step 7
Give two ways in which you would expect the melting point of the impure benzoic acid to differ from that of the purified acid.
Answer
The melting point of the impure benzoic acid is expected to be lower than that of the purified acid due to the presence of impurities which disrupt the orderly packing of the crystals. Additionally, the melting point of the impure sample may also be broader, indicating a less-defined melting range compared to the sharp melting point of the pure benzoic acid.
Step 8