2-Naphthol has solubility in water = 0.07 g/100 mL at 20 degree C and 0.18 g/100

Question

2-Naphthol has solubility in water = 0.07 g/100 mL at 20 degree C and 0.18 g/100 mL at 100 degree C. Assume that recrystallization of 1.0 g of crude benzoic acid comprised 10% of 2-naphthol, predict whether the fate of the 2-Naphthol impurity. (Show calculations to support your response.) Would it be removed as an insoluble impurity by hot filtration? Rationalize. Would it be a soluble impurity that is removed by cold filtration? Rationalize. Would it contaminate the recrystallized benzoic acid since it would co-recrystallize with the benzoic acid? Rationalize.

Explanation / Answer

As per the assumption, our recrystallised product comprises of 0.1g 2-Naphthol and 0.9g Benzoic acid.

a. For the hot filteration, at high temperature(100 0C) 0.18g of 2-Naphthol is present in our solution. Since only 0.1 g recrystallises, we are left with 0.18 - 0.1 = 0.08g of 2-Naphthol in solution. This 0.08g of 2-Naphthol will be soluble at hot filteration and therefore will not be removed by hot filteration.

b. For the cold filteration, at 20 0C only 0.07g of 2-Naphthol will be soluble in the solution. So the remaining 0.08 - 0.07 = 0.01g will be solid which can be removed by filteration.

Thus, 2- Naphthol will be a soluble impurity which is not completely removed by cold filteration.

c. The soluble 2-Naphthol present in the solution will co-recrystallise with the benzoic acid and thus contaminate the recrystallised benzoic acid.

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