A mixture contains solid benzoic acid and solid sodium chloride. Which sequence of steps is most appropriate to separate and purify the benzoic acid?
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Correct answer
D. Add water, filter, then recrystallize the filtrate
Principle or equation
Benzoic acid is sparingly soluble in cold water but more soluble in hot water; sodium chloride is very soluble. Dissolving in hot water, filtering to remove insoluble impurities, then cooling to recrystallize benzoic acid.
Why this answer is correct
Add hot water to dissolve both. Benzoic acid dissolves in hot water; NaCl also dissolves. The mixture is filtered hot to remove insoluble impurities. Upon cooling, benzoic acid crystallizes and can be collected by filtration. The residue after initial filtration would be insoluble impurities, not benzoic acid.
Example
If you have 5 g of benzoic acid and 5 g of NaCl, add 100 mL hot water, filter hot, cool the filtrate to precipitate benzoic acid, then filter and dry.
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