1. Five grams of the crystalline compound “A” dissolve in 100 mL of acetone at r
ID: 714385 • Letter: 1
Question
1. Five grams of the crystalline compound “A” dissolve in 100 mL of acetone at room temperature. At the boiling point temperature of acetone, 50 grams of the same compound dissolve. When recrystallizing 100 g of compound A from acetone, how much solid material can theoretically be isolated after cooling to room temperature when the following volumes of solvent (acetone) are used: a) 100 ml, b) 200 mL, or c) 300 mL? Would the purity of the three products differ? Please elaborate.
2. What is the approximate temperature of a 0.01 molar solution of benzoic acid in ethanol that is heated in a boiling water bath (100 deg celcius) ?
Explanation / Answer
Q1) Given,
Solubility of compound A at room temperature (RT) = 5 g / 100 mL = 0.05 g/mL
Solubility of compound A at boiling temperature (BT) of acetone = 50 g / 100 mL = 0.5 g/mL
Mass of compound A taken for recrystallization = 100 g
Case a) we are dissolving 100 g of compound A in 100 mL of acetone at BT. So maximum it can dissolve is 50 g of compound A. Because at BT solubility is 50 g / 100 mL. So the remaining 50 g of compound A will remain undissolved. This is oversaturated solution.
Hence while recrystallizing the undissolved compound A will remain as it is and will be technically impure.
After cooling to RT the amount of compound A remaining in solution is = 5 g / 100 mL = 5 g.
This 5 g will not be recovered (unless we reduce the volume by distillation!).
Hence, amount we can recover = amount undissolved + amount dissolved - amount remaining in solvent = 50 + 50 - 5 = 95 g.
The recrystallized compound A will be impure.
Case b) we are dissolving 100 g of compound A in 200 mL of acetone. So maximum it can dissolve is 100 g of compound A. So this is the saturated solution. No compound A is remaining undissolved.
After cooling to RT amount remaining in solvent = 10 g because total volume is 200 mL and solubility is 5g / 100 mL.
This 10 g will not be recovered.
Amount we can recover in recrystalization = amount undissolved + amount dissolved - amount remaining in solvent = 0 + 100 - 10 = 90 g.
The recrystallized compound A will be pure.
Case c) we are dissolving 100 g of compound A in 300 mL of acetone. So maximum it can dissolve is 150 g of compound A. So this is the unsaturated solution. Which is difficult to recrystallize. And the yield will be very low.
After cooling to RT amount remaining in solvent = 15 g because total volume is 300 mL and solubility is 5g / 100 mL.
This 15 g will not be recovered.
Amount we can recover in recrystallization = amount undissolved + amount dissolved - amount remaining in solvent = 0 + 100 - 15 = 85 g
The recrystallized compound A will be pure but with low yield.
Q2. The boiling point of ethanol is 78oC. Hence the maximum temperature, the solution can attain is 78oC even the temperature of the water bath may be 100oC.
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