How Do You Purify Compounds Using Recrystallization Techniques?

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SUMMARY

This discussion focuses on the purification of compounds using recrystallization techniques, specifically addressing the challenges of separating compound A from compound B. The solubility of compound A in ethyl acetate is 18 mg per 50 mL at 20°C and 0.28 g per 50 mL at 75°C. It is established that while recrystallization can yield a purer product, complete purity is not achievable due to the potential co-precipitation of contaminants. The discussion also includes calculations for the minimum amount of ethanol required to recrystallize compound C, which has a solubility of 24 mg per 100 mL at 20°C and 0.78 g per 50 mL at 90°C.

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  • Understanding of recrystallization techniques in organic chemistry
  • Knowledge of solubility principles and temperature effects
  • Familiarity with mixed melting point analysis
  • Basic skills in stoichiometric calculations for solubility
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Chemistry students, laboratory technicians, and researchers involved in organic synthesis and compound purification will benefit from this discussion.

quarkboy
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Recrystallization Questions - help needed urgent!

I can't figure this out, please help... I will update with answers as I get them.

1. Compound A has a solubility in ethyl acetate of 18 mg per 50 mL at 20 Celsius, and a solubility of 0.28g per 50 mL at 75 Celsius.

You are given a sample of 0.800g compound A which is contaminated with 2 mg of compound B.

(a). If compound B is always completely soluble in ethanol, how could you purify compound A? Would the result of this purification be 100% pure A? Explain.

Ans: Recrystallization method must be followed as per lab practice. However, since compound B dissolves completely in the solvent we are able to obtain a pure end product.

(b). If compound B has approx. the same values of solubility in ethyl acetate as compound A, how could you purify compound A? Would the result of this purification be 100% pure A? Explain.

Ans: As above, however mixed melting points must be used to identify the compound A. It will not be completely pure as an end-product.

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3. The solubility of an unknown compound, C, in ethanol is 24mg per 100 ml at 20 Celsius and 0.78g per 50ml at 90 Celsius.

(a). Calculate the min. amount of ethanol needed to recrystallize 2.4g of compound C.

Ans: 0.78g/50ml * 0.024g/x ; x = 158.3ml of ethanol required [90 Celsius]

(b). Calculate the amount of compount C recovered by filtration at 20 Celsius from this min. amount of ethanol.

Ans: 158.3ml (as above) * 24mg/100ml = y

Therefore, 2.4g - y = amount of ethanol [20 Celsius]
 
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Quarkboy, please show what effort you've made first. Click my signature.
 
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Thanks.

Refer to Original Post.
 
(a). If compound B is always completely soluble in ethanol, how could you purify compound A? Would the result of this purification be 100% pure A? Explain.

Ans: Recrystallization method must be followed as per lab practice. However, since compound B dissolves completely in the solvent we are able to obtain a pure end product.

How soluble is compound A in ethanol? btw, recrystallization procedures are basically never 100% pure, in order to obtain such purities (such as with hydrogen phthalate) one needs to adhere to additional procedures (for instance, drying, your product will probably be contaminated with water). Crystallization always involves a bit of co-precipitation, Fajan's method actually depends on such a phenomenon.
 
quarkboy said:
I can't figure this out, please help... I will update with answers as I get them.

1. Compound A has a solubility in ethyl acetate of 18 mg per 50 mL at 20 Celsius, and a solubility of 0.28g per 50 mL at 75 Celsius.

You are given a sample of 0.800g compound A which is contaminated with 2 mg of compound B.

(a). If compound B is always completely soluble in ethanol, how could you purify compound A? Would the result of this purification be 100% pure A? Explain.

Ans: Recrystallization method must be followed as per lab practice. However, since compound B dissolves completely in the solvent we are able to obtain a pure end product.

(b). If compound B has approx. the same values of solubility in ethyl acetate as compound A, how could you purify compound A? Would the result of this purification be 100% pure A? Explain.

Ans: As above, however mixed melting points must be used to identify the compound A. It will not be completely pure as an end-product.
Looks roughly okay so far...but it's never an easy task to guess what the teacher "wants" you to say.


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3. The solubility of an unknown compound, C, in ethanol is 24mg per 100 ml at 20 Celsius and 0.78g per 50ml at 90 Celsius.

(a). Calculate the min. amount of ethanol needed to recrystallize 2.4g of compound C.

Ans: 0.78g/50ml * 0.024g/x ; x = 158.3ml of ethanol required [90 Celsius]
That's not an equation. Please rewrite the solution showing your reasoning. Where have you used the fact that you want to make 2.4 g of C crystals ?

How much C will dissolve in 100 mL at 90C ? How much at 20C ? What happens to the difference ? This tells you the weight of crystal formed per 100mL of ethanol. Now use this ratio to determine the volume of ethanol required to make 2.4g of crystal.


(b). Calculate the amount of compount C recovered by filtration at 20 Celsius from this min. amount of ethanol.

Ans: 158.3ml (as above) * 24mg/100ml = y

Therefore, 2.4g - y = amount of ethanol [20 Celsius]
Redo above part and then retry this.
 

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