Explain Why The Percent Recovery Is Frequently Low
1. A student neglected to add sodium carbonate when extracting the tea leaves with hot water. Yet a very high yield of caffeine was obtained. The crystals melted at 202-214°C. Explain.
• The sodium carbonate is supposed to be added when extracting the tea leaves with hot water in order to help purify the caffeine. So since the student neglected to add sodium carbonate the caffeine won’t be pure but the student would still be able to recover a large amount of caffeine. The literature melting point is 238°C. Because the caffeine wasn’t pure enough, the observed melting point was a little off.
2. The weight of the purified caffeine is often much lower than the weight of the crude caffeine. Explain why the percent recovery is frequently low.
• Crude caffeine contains contaminants, such as organic compounds, while purified caffeine is just pure caffeine. Therefore, the weight of the crude caffeine is more than the purified caffeine because of the additional contaminants adds weight. Since there are no additional contaminants to the purified caffeine, the percent recovery would be low. The percent recovery would also be low since the caffeine is pure and does not contain any contaminants.
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Cholesterol is purified using a hot gravity filtration. Why must the beaker and filter paper be kept hot?
• Hot gravity filtration is used to separate insoluble impurities from a hot solution. Both the beaker and the filter paper should be kept hot in order to stop the dissolved compounds crystallizing from solution during filtration, thus forming crystals on the
filter.
4. Is it better to do two extractions with 10 mL of methylene chloride or one extraction with 20 mL of methylene chloride? Explain.
• Two extractions with 10-mL methylene chloride is better than one extraction with 20-mL methylene chloride because the more extractions you do, the more amount of solvent you have. Doing multiple extractions ends up having more than equilibrium stage. The distribution coefficients for each separate extraction can then be added together which would then yield a higher percent recovery.
5. Identify all of the functional groups present in cholesterol.
• Cholesterol has four hydrocarbon rings that have alkane, alkene, and alcohol (hydroxyl) functional groups present.
• The sodium carbonate is supposed to be added when extracting the tea leaves with hot water in order to help purify the caffeine. So since the student neglected to add sodium carbonate the caffeine won’t be pure but the student would still be able to recover a large amount of caffeine. The literature melting point is 238°C. Because the caffeine wasn’t pure enough, the observed melting point was a little off.
2. The weight of the purified caffeine is often much lower than the weight of the crude caffeine. Explain why the percent recovery is frequently low.
• Crude caffeine contains contaminants, such as organic compounds, while purified caffeine is just pure caffeine. Therefore, the weight of the crude caffeine is more than the purified caffeine because of the additional contaminants adds weight. Since there are no additional contaminants to the purified caffeine, the percent recovery would be low. The percent recovery would also be low since the caffeine is pure and does not contain any contaminants.
3. …show more content…
Cholesterol is purified using a hot gravity filtration. Why must the beaker and filter paper be kept hot?
• Hot gravity filtration is used to separate insoluble impurities from a hot solution. Both the beaker and the filter paper should be kept hot in order to stop the dissolved compounds crystallizing from solution during filtration, thus forming crystals on the
filter.
4. Is it better to do two extractions with 10 mL of methylene chloride or one extraction with 20 mL of methylene chloride? Explain.
• Two extractions with 10-mL methylene chloride is better than one extraction with 20-mL methylene chloride because the more extractions you do, the more amount of solvent you have. Doing multiple extractions ends up having more than equilibrium stage. The distribution coefficients for each separate extraction can then be added together which would then yield a higher percent recovery.
5. Identify all of the functional groups present in cholesterol.
• Cholesterol has four hydrocarbon rings that have alkane, alkene, and alcohol (hydroxyl) functional groups present.