3. What VOLUME (mLs) of 95° water is needed to dissolve 10 grams of benzoic acid

Question

3. What VOLUME (mLs) of 95° water is needed to dissolve 10 grams of benzoic acid? (Use solubility data in text). (A) If this solution is cooled to 10°C and recrystallized, how many GRAMS of benzoic acid will crystalize? (B) What is the maximum percent recovery? 4. How is activated carbon used in the recrystallizing process? Why should a large excess of activated carbon be avoided? 5. Under what circumstances is a miscible solvent mixture more effective than a single solvent in the recrystallization process? 48

Explanation / Answer

Ans 3 :

The solubility of benzoic acid at 95oC is 6.8 g per 100mL of water and at 10o C is 0.21 g per 100 mL of water.

So when 6.8 g of it dissolves in 100 mL

Then 10 g will dissolve in ( 10 x 100) / 6.8

= 147 mL of water

a) 100 mL of this solution dissolves 0.21 g of benzoic acid at 10 degrees C

So 147 mL of this solution , will dissolve ( 147 x 0.21) / 100

= 0.31 g of benzoic acid

So the amount that recrystallises = 10 g - 0.31 g = 9.69 g

b) Maximum percent recovery = (mass recovered / mass initially ) x 100

= ( 9.69 / 10) x 100

= 96.9 %

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