Question

1. You are given a mixture containing two compounds, A and B. Both compounds have a solubility of 1 g/ 100 mL of solvent at 20 °C and 16 g/ 100 mL of solvent at 100 °C. The sample is composed of 3.5 g of A and 10 g of B. At 100 °C the entire sample just dissolves in a minimum amount of solvent. The solution is cooled to 20 °C and crystals are collected. Calculate the composition of the crystals and the yield of the process. What is the composition of the mother liquor?
2. If the crystals obtained in question (2) are recrystallized from 100 mL of solvent, what will be the yield and composition of the crystals obtained?

Answer 1

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Answer 2

At 100 degrees celsius Compound B (Has the greater mass): 10g x 100mL/16g = 62.5mL When cooled to 20 degrees celsius Compound A: 62.5mL x 1g/100mL = 0.625g Compound B: 62.5mL x 1g/100mL = 0.625g Therefore the mother liquor contains 0.625g of each. Composition of crystal Compound A = 3.5g - 0.625g = 2.875g Compound B = 10g – 0.625g = 9.375g Percent Yield Compound A = 2.875g/(2.875+9.375)g x 100% = 23.5% Compound B = 9.375g/(2.875+9.375)g x 100% = 76.5% % yield of process = (2.875+9.375)g/(3.5+10)g x 100% = 12.5g/13.5g x 100% = 90.7%