Question

Question 1 A sample of naphthalene, which should be pure white, was found to have a grayish color after the usually purification procedure. The melting point was correct, and the melting point range was small. Explain the gray color.

Question 2 How many millimeters of boiling water are required to dissolve 25 g of phthalic acid? If the solution was cooled to 14*C, how many grams of phthalic acid would recrystallize out?

Question 5 Under which circumstance is it wise to use a mixture of solvents to carry out a recrystallization?

Answer 1

Q. 5

Recrystalization is a technique used to purify chemical substances.

In this technique material to be purified is dissolved in appropriate hot solvent (the solvent must be insufficient to dissolve whole compound, should not react with the compound, and should be non-flammable, inexpensive and volatile). When above solution cooled, crystals of pure substances formed leaving impurities out of crystal lattice in solution, thereby completing the purification process. Then the solution is filtered to get pure compound and (impurities in filtrate) can be washed further with small amount of solvent to enhance purity.

Solvent has crucial role in recrystalization. It must not dissolve the compound at low temperatures but must dissolve the compound at high temperatures. Conversely, it should not dissolve impurities readily or not at all in cold solvent otherwise would the desired product and the impurities both crystallize simultaneously from solution.

Mixture of solvents is used in case when both the desired product and impurity are dissolved in the first solvent. In this case instead of hot filtration process (by temperature difference technique as in above case) separation is done by using more than one solvent

Since both the desired product and impurity are dissolved in the first solvent another solvent is added, in which the desired product or impurity will be insoluble and would precipitate. The addition of second solvent must be slow to reach crystallization point and cool the solution once compound begin to crystallize.

For example if your mixture is soluble in a polar solvent. Then dissolve your compound in polar solvent first (saturation point) and then add non polar solvent slowly, either impurity or product will be crashed out of mixture.