Question

Benzoic acid’s solubility in H2O is 68.0 g/liter at 95°C & 1.7 g/liter at 0°C. 1). What is the minimum amount of H2O needed to recrystallize a 9.68 g sample of benzoic acid at 95°C? This represents the solubility in the hot solvent. 2) How many grams of benzoic acid will remain in solution at 0 C after the crystals are collected? This represents the solubility in the cold solvent. 3) How many grams of pure benzoic acid will be collected after crystallization is complete?

4) Calculate the % lost in the cold solvent using the formula: % lost in cold solvent = [sol. in cold solvent / sol. in hot solvent] x 100

5) Calculate the % recovery of solid using the formula: % recovery solid = [g (solid) i - g(solid lost)] / g (solid) i x 100; where i = initial

Answer 1

1) To dissolve 9.68g of sample at 95 degrees, water required = 1 x 9.68/68 L

= 0.14L

2) When cooled to 0degrees, the 0.14 L will dissolve Benzoic Acid = 0.14 x 1.7 g of benzoic acid = 0.242 grams. Therefore, 0.242 grams of the original 9.68 g will remain in solution.

3) 0.242g of benzoic acid dissolved in the solution would be collected.

4) % lost in cold solvent = 0.242/9.68 *100 = 2.5%

The theoretical maximum you can recover under these conditions is ten minus .441 grams, or 9.559 grams.When we dissolve the