On the other hand, in reaction (3), the HCl added should be in excess to ensure maximum benzoic acid formation. The sign of complete reaction can be told by testing it with wet blue litmus paper, it should turn from blue to red, indicating that the reaction mixture has become acidic.
Moreover, since benzoic acid has very low solubility in water, white precipitate would form which can be separated by simple filtration. Also a suction pump is connected below to speed up the filtration. The filtrate should then be washed to remove any impurities.
Lastly, by weighing the initial and final mass of the benzamide and benzoic acid respectively, the percentage yield of the process can be calculated. Moreover, by testing the melting point of the resulting benzoic acid solid with the melting point apparatus, the purity of the product obtained can be estimated, since the purer the benzoic acid, the sharper the melting point and the nearer to its theoretical value (~126°C)
- Results:
Initial mass of benzamide : 2.99 g
Initial no. of moles of benzamide = 2.99 / (12x7+1x7+14+16)
= 0.025 mol
Final mass of benzoic acid : 0.74 g
Final no. of moles of benzoic acid : 0.74 / (12x7+1x6+16x2)
= 0.006 mol
Percentage yield = 0.006/0.025 = 24.00 %
Melting point determined = 123 ~ 127°C
Maximun Percentage error = (123-126)/126 x 100% = -2.38%
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Discussion:
Sources of Error
- Imurities was present in the original benzamide powder, hence the initial mass obtained may be higher that expected, thus the percentage yield calculated is lower than the actual one.
- Some benzamide as well as benzoic acid powder may have lost during transfer, resulting in a low percentage yield.
- Some benzoic acid solid may have dissolved in water during filtration and was discarded as filtrate, resulting in a loss of product and hence a low percentage yield.
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Conclusion:
The percentage yield of this process is 24.00% while the melting point of the resulting benzoic acid solid ranges from 123 to 127°C, with a percentage error of -2.38%
