The table below provides the solubilities of compounds A and B in ethanol. You a

Question

The table below provides the solubilities of compounds A and B in ethanol. You are purifying 10 g of compound A contaminated with 1 g of compound B. (a) Calculate the minimum volume of ethanol that you would need to recrystalize the sample. (b) Calculate the % Recovery of A upon recrystallization, disregarding experimental losses. (c) Is A pure after recrystallization? (b) Is A pure after recrystallization if A (10 g) is contaminated with 1.5 g of B? What is the % of A in the recrystallized product?

Explanation / Answer

a)

The solubility of compound A in C2H5OH is 7 g in 100ml at the boiling point

100mL of ethanol is needed just to dissolve 7.0g of compound A

Volume of ethanol needed to dissolve 10.0g of compound A = 10.0g x 100ml/7.0g = 142.86ml

142.86mL of ethanol is needed for recrystallization of 10.o g of compound A at boiling point.

b) In 100 mL, at 0 °C, it dissolves only 0.1 g of A,

So in 142.86ml at 0 °C, it dissolves 0.1 g x 142.86ml/100ml = 0.143 g of A

0.143 g of compound A is still in ethanol.

Recovered mass of compound A after recrystallization = 10.0g – 0.143g = 9.857g

% Recovery of compound A = (recovered mass of compound A/original mass of compound A) x 100

% Recovery of compound A = (9.857g/10.0 g) x 100 = 98.57%

c) Yes, compound A is pure after recrystallization

Solubility of compound B in ethanol at 00C = 0.9g/100ml

In 100ml of ethanol 0.9 g of compound B is soluble

So in 142.86 ml of ethanol 0.9 g x 142.86 ml/100 ml = 1.28g of compound B is soluble

There is 1g of compound B, which is soluble in 142.86 ml of ethanol.

d) No, compound A is not pure after recrystallization, when 10g of compound A is contaminated with 1.5g of compound B.

1.28g of compound B is soluble in soluble in 142.86 ml of ethanol

1.5 – 1.28 = 0.22g of compound B is recrystallization

Total recrystallized product = 9.857g + 0.22g = 10.077g

% of A in recrystallized product = (9.857g /10.077g) x 100

% of A in recrystallized product = 97.8%

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