Solve Recrystalization Lab Homework - Comp. & Yield of Crystals, Mother liquor

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In summary, a mixture of compounds A and B with solubility of 1g/100mL at 20 degrees Celsius and 12g/100mL at 100 degrees Celsius is composed of 1.5g of A and 10g of B. When cooled from 100 degrees Celsius to 20 degrees Celsius, the crystals collected have a composition of 13.05% A and 86.95% B. The yield of the process is also calculated from these percentages. The composition of the mother liquor is unclear.
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mirandasatterley
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Homework Statement



You are given a mixture containing two compounds, A and B. Both compounds have a solubility of 1g/100mL of solvent at 20 degree celcius and 12g/100mL of solvent at 100 degree celcius. The sample is composed of 1.5g of A and 10g of B. At 100 degree celcius all of the sample just dissolves in a minimum amount of solvent. The solution is cooled to 20 degree celcius and crystals are collected. Calculate the composition of the crystals and the yeild of the process. What is the composition of the mother liquor?

Homework Equations



No equations.

The Attempt at a Solution



Compound A: 1.5g x 100mL/12g = 12.5mL
Compound B: 10g x 100mL/12g = 83.3mL
So from this, of the solution (solvent that just dissolves A and B), 12.5mL dissolves A and 83.8mL dissolves B.

When cooled, the solubility is only 1g/100mL, so
Compound A: 12.5mL x 1g/100mL = 0.125g
Compound B: 83.3mL x 1g/100mL = 0.833g
Therefore the crystals collected have composition:
Compound A = 0.125g/(0.125 +0.833)g x 100% = 13.05%
Compound B = 0.833g/(0.125 +0.833)g x 100% = 86.95%

Does any of this make sense? If not, where did I go wrong? I'm also unsure of how to calculate the yeild of the process and the composition of the mother liquor.

Any help is greatly appreciated. Thanks.
 
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  • #2
You already calculated the yield when you figured out the %'s of compound A and B...as for the composition of mother liquor, I have no clue what its asking for.
 
  • #3




Your calculations for the composition of the crystals seem correct. However, to calculate the yield of the process, you need to know the total amount of crystals collected. Without this information, it is not possible to determine the yield. As for the composition of the mother liquor, it would contain the remaining amount of both compounds A and B, which would be 1.375g and 9.167g respectively. This can be calculated by subtracting the amounts of compounds A and B in the crystals from the original amounts of 1.5g and 10g respectively. I hope this helps clarify things for you.
 

1. What is recrystallization and why is it important in a lab setting?

Recrystallization is a technique used to purify solid compounds in a laboratory setting. It involves dissolving the compound in a solvent, then allowing it to cool and form crystals, which can then be separated from impurities. This process is important because it can remove impurities and improve the overall purity of a compound, making it suitable for further experiments or use in various applications.

2. How do you calculate the percent yield of crystals in a recrystallization experiment?

The percent yield of crystals is calculated by dividing the actual yield (the amount of crystals obtained from the experiment) by the theoretical yield (the maximum amount of crystals that could be obtained based on the starting material used), and then multiplying by 100. This calculation gives an indication of the efficiency of the recrystallization process.

3. What is the purpose of using a mother liquor in recrystallization?

A mother liquor is the leftover solution after the crystals have formed and been removed in a recrystallization experiment. It contains impurities and residual solvent that were not incorporated into the crystals. The purpose of using a mother liquor is to prevent these impurities from contaminating the final product, as the crystals can be washed with fresh solvent and then dried to improve their purity.

4. Can any solvent be used for recrystallization?

No, not all solvents are suitable for recrystallization. The solvent used should be able to dissolve the compound at high temperatures, but not at room temperature. It should also have a high boiling point, be relatively non-toxic, and be easily separable from the compound after the crystals have formed. Common solvents used for recrystallization include water, ethanol, and acetone.

5. How can you determine the purity of the crystals obtained from a recrystallization experiment?

The purity of crystals can be determined by performing a melting point test. Pure crystals have a sharp and consistent melting point, while impure crystals will have a lower and less defined melting point due to the presence of impurities. Additionally, techniques such as thin-layer chromatography or spectroscopy can also be used to analyze the purity of crystals.

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