How many milliliters of boiling water are required to dissolve 25 g of a sugar (

Question

How many milliliters of boiling water are required to dissolve 25 g of a sugar (sucrose)? After cooling to 20 degree C, how much sugar would crystallize out? (Solubility data: at 20 degree C: 203 g sugar/100 mL water; at 100 degree C 487 g sugar/100 mL water). Show all calculations Why is it necessary to use a minimum amount of boiling solvent and not a larger amount? A sample of (after on using decolorizing charcoal and hot filtration) was found to have a grayish color. The melting point was identical to the reported melting point of naphthalene and the range was correspondingly small. Why are the melting points identical even though the sample is discolored? Why use gravity filtration to remove solids in a hot filtration and not vacuum filtration? Why use vacuum filtration to filter the final product instead of gravity filtration? Why can we use water-ethanol as a recrystallization solvent but we cannot use cyclohexane-water as a recrystallization solvent?

Explanation / Answer

8.1

Solubility of sucrose in water at 100 oC = 487 g/100 ml water

So,

to dissolve 25 g sucrose we would need = 25 x 100/487 = 5.13 ml

Solubility of sucrose in water at 20 oC = 203 g/100 ml water

So when cooled at 20 oC, amount recrystallize out = 25 - (25 x 100/203) = 12.69 g

8.2

If more water is used, it would remain in dissolved state in solution and would not recrystallize out when cooled.

8.3

Colored impurities do not add to the melting change changes of a compound to much extent. Therefore the melting point is identical even when sample is discolored as other impurities had been removed.

8.4

Gravity filtration removes insoluble impurities. Under vacuum the solvent may start to evaporate and recrystallization would occur during filtration itself, which is not preferred and thus not used. Product is filtereted quickly using vacuum filtrarion as higher times on filter may result into redissolution of solid into the solution.

8.5

Cyclohexane is a non-polar solvent and thus cannot be a substitute for a polar solvent as ethanol is recrystallization.

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