Data and Calculation
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The gas evolved in step 4: NH3
Test: the steam was directed to a piece of pH paper to measure the pH. The pH measured was 9.
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Color of the crystals obtained after recrystallization: white
Shape of the crystals: needle-shaped
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Mass of banzamide + weighing bottle: 7.81g
Mass of weighing bottle: 4.65g
Mass of benzamide: 7.81 – 4.65 = 3.16g
Number of moles of benzamide: 3.16/ (109) = 0.0290moles
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Weight of crystals + watch glass: 27.808g
Weight of watch glass: 26.546g
Weight of crystals: 27.808 – 26.546 = 1.262g
Number of moles of benzoic acid: 1.262/ (122) =0.0103 moles
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Percentage yield: 0.0103/ 0.0290 = 35.7%
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Melting point of the crystals: 120~126oC
Conclusion
- The percentage yield of pure benzoic acid produced is 35.7%.
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The melting point of the benzoic acid ranges from 120~126oC. The acid is rather impure.
Discussion
- The loss of benzoic acid comes from the following sources:
- The reaction mixture was boiled twice. The first time was for dissolving benzamide to hydrochloric acid, and the second one was for recrystallization. Some of the benzoic acid was turned into vapour, which results in a loss of materials.
- While transferring the substances to different apparatus, not all of the substances were transferred during the process.
- Not all benzoic acid were crystallized during recrystallization.
- A wide melting point range implies that the acid is impure. This is due to several reasons:
- Recrystallization was ineffective.
- While being left for 1 night, the acid was exposed in air and some impurities may enter the acid crystals.
- In this experiment, the method of recrystallization was used to purify the benzoic acid obtained. But the solvent used should be chosen with care. Consider the following graph which gives the solubilities of 4 solvent at different temperatures.
In the graph, solvent A should be the most suitable as it has a low solubility at low temperatures and a high solubility at high temperatures. Hence, more crystals can be formed when the temperature goes down.
- The standard method of arranging the Buchner funnel is as follows:
The opening of the tip of the funnel should be facing the suction pump for effective suction.
- One precaution must be taken while using this pump. When the suction filtration is to be stopped, either the Buchner funnel is pulled of or the rubber tubing is removed. The water pump should not be turned off directly, otherwise there will be sucking back.
- The purpose of determining the melting point is not just to determine the melting point so simple. It also aims at measuring the purity of the crystals obtained. If the crystals are pure, the crystals should have a sharp melting point.
- There are also other means of measuring the purity of the crystals. First, determine the melting point of the acid with the data book. Then, either
- Mix the product with pure benzoic acid and see if the melting point of the mixture is consistent with that found in the data book. Otherwise, the melting point should be lower than this value because impurities would depress the melting point of the mixture.
- Carry out further recrystallization to see whether the products obtained have a consistent melting point.
8. The method of recrystallization is not suitable for purification of every reaction. The criterion required is that the substance to be purified has a much lower solubility than the impurities formed. For example, in this experiment, benzoic acid is a lot less soluble than the main impurity, ammonium. So, benzoic acid is suitable for purification using this method.