# Homework Help: Recrystallization Questions - help needed !

1. Oct 6, 2005

### quarkboy

Recrystallization Questions - help needed urgent!

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]

Last edited: Oct 7, 2005
2. Oct 7, 2005

### Gokul43201

Staff Emeritus

Last edited: Oct 7, 2005
3. Oct 7, 2005

### quarkboy

Thanks.

Refer to Original Post.

4. Oct 7, 2005

### GCT

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.

5. Oct 7, 2005

### Gokul43201

Staff Emeritus
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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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.

Redo above part and then retry this.