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Q1: Why is dichloromethane used to dissolve the mixture in the cellulose recovery step?
Dichloromethane (DCM) selectively dissolves caffeine and benzoic acid while leaving cellulose insoluble. This solubility difference allows you to separate cellulose from the other compounds through filtration. The insoluble cellulose remains on the filter paper while the dissolved compounds pass through into the filtrate.
Q2: How does sodium hydroxide help separate benzoic acid from caffeine?
Sodium hydroxide converts benzoic acid into sodium benzoate, which is highly water-soluble and insoluble in DCM. This chemical transformation allows the aqueous and organic layers to separate, with sodium benzoate moving into the water layer while caffeine remains in the DCM layer, enabling effective separation of these two compounds.
Q3: What is the purpose of magnesium sulfate in the organic layer?
Magnesium sulfate acts as a drying agent to remove residual water from the organic layer containing caffeine. You add it until a portion remains white and powdery, indicating it has absorbed available moisture. This drying step ensures the purity of the recovered caffeine by removing water before evaporation.
Q4: Why is the aqueous solution chilled before adding hydrochloric acid?
Chilling the aqueous solution in an ice bath lowers the solubility of benzoic acid in water. When hydrochloric acid reprotonates the benzoate anion back to benzoic acid, the cold temperature promotes precipitation of the solid compound, making it easier to collect by filtration and improving recovery yield.
Q5: What does the pH paper measurement indicate during the reprotonation step?
The pH paper confirms that sufficient hydrochloric acid has been added to convert all sodium benzoate back to benzoic acid. The target pH range of 1–3 ensures complete reprotonation. If the pH is too high, benzoate remains in solution; if too low, excess acid contaminates the product.
Q6: How do you calculate the percent yield for each recovered compound?
Percent yield is calculated by dividing the recovered mass of each compound by its theoretical initial mass, then multiplying by 100. The theoretical mass is determined from the known composition of the mixture (5% cellulose, 47.5% caffeine, 47.5% benzoic acid) multiplied by your starting sample mass.
Q7: What does it mean if the sum of recovered compound masses exceeds the starting mixture mass?
If the total recovered mass is greater than the starting mass, one or more compounds retained residual moisture when weighed. You should reweigh the compounds after allowing them to dry completely. Accurate mass measurements require all solids to be completely dry to ensure valid percent yield calculations.