Question

MW (g/mol) / MP ( oC) / Solubility in cold H2O (g / 100 mL) / Solubility in boiling H2O (g / 100 mL)

Acetanilide 135.2 / 114 / 0.54 / 5.0

Phenacetin 179.2 / 135 / 0.076 / 1.22

Using solubility data in the table of reagents, calculate the minimum amount water needed to recrystallize:

i. 1.5 g of phenacetin.

ii. 1.5 g of acetanilide.

b. If the minimum amount of water needed is used, how many grams of each compound would crystallize out of solution following the recrystallization?

i. phenacetin

ii. acetanilide

Answer 1

(a)

The solubility of phenacetin in hot water is 1.22 g / 100 ml of hot water.To calculate minimum amount of water needed to recrystallize, we just invert the solubility data.Recrystallization start when boiling start

So minimum amount of water = (gram of compound) / (solubility in hot water)

(i) 1.5 g phenacetin

minimum amount of water = (1.5) / ( 1.22 / 100 ml of water) =(1.5*100/1.22) =122.95 ml

Same way for other product

(ii) 1.5 g acetanilide

minimum amount of water = (1.5) / ( 5 / 100 ml of water) =(1.5*100/5) =30 ml

(b)

(i) phenacetin

At lower temperature the crystall is dissoved in water.

So amount of dissolved compound =minimum water*solublityin cold water

=122.95*0.076/100=0.093442 gm

So Crystallize compund out of solution = 1.5-0.093442=1.4065 gm

Same way

(ii) acetanilide

amount of dissolved compound =minimum water*solublityin cold water

=30*0.54/100=0.162 gm

So Crystallize compund out of solution = 1.5-0.093442=1.338 gm