a. For effective purification by recrystallization the compound must be much mor

Question

a. For effective purification by recrystallization the compound must be much more soluble in ___________ solvent than in ____________ solvent, and the impurities must be either_____________ in hot solvent or _____________ in cold solvent.

b. Why must the filter paper fit flat on the bottom the Hirsch funnel and be wet before beginning collection of crystals?

c. The solubility of acetanilide in hot water (5.5 g / 100 mL at 100 °C) is significant and its solubility in cold water (0.53 g / 100 mL at 0 °C) is not very great. What would be the maximum theoretical percent recovery (first crop only) from the crystallization of 3.4 g of acetanilide from 65 mL of water (assuming the solution is chilled to 0 °C)?

d. What happens if you cool the recrystallization solution in a container of ice that is too big for the crystallization flask?

Explanation / Answer

1) For effective purification by recrystallization the compound must be much more soluble in ___hot________ solvent than in ____cold________ solvent, and the impurities must be either____filtered_________ in hot solvent or ____vaccum filtered_________ in cold solvent.

b)  to keep crystals from escaping under the filter paper into the filter flask while filtering

c)

0.53 g acetanilide / 100 mL water = x g acetanilide / 65 mL water.
Doing the algebra, x = 0.34 g acetanilide. This is the amount that actually stays in the water. The amount of acetanilide that crystallizes out is:3.4 g - 0.34 g = 3.06g
Therefore, the maximum percent recovery is:
(3.06 g) / (3.4 g) = 90.0%.

d)Crystallization is a physical change in which a substance changes from liquid form to solid form. A supersatured solution is used for this purpose.Then, when you cool it down in an ice batj, solid crystals will form. The bigger the container the ice, the faster is the rate of crystallization and thus bigger the size of solid crystals would be.

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