Question

A solid M is soluble in water to the extent of 14.0 mg / mL of water at 25 ℃ and 110. mg / mL at 100 ℃. You have a the sample that contains 132 mg of M along with 24.0 mg of impurity N, which displays the same solubility behavior in the solvent as compound M.

 i) By showing ALL the calculation steps, describe how you would be able to purify M if impurity N has the same solubility behavior as compound M in water.

 ii) Amount of pure M isolated after all the crystallization steps = _______ 

 iii) Number of crystallization steps = _______ 

 iv) Calculate the percent recovery of pure solid M after all the crystallization steps.

Answer 1

Firstly boil the minimum amount of water (solvent) and dissolve the sample into it . 110mg of M and all 32mg of N will dissolve in it at 100°C . Now cool the solution to room temperature ( crystallisation) . Now 118mg of M along with 10mg of impurity N ( as 14mg of both M and N is soluble at 25°C too i.e at room temperature ) will be precipitate out as crystals .

Thus , amount of pure M isolated afrer crystallisation = 118mg

No. Of crystallisation steps = 1 ( only one step is possible practically with water in given conditions) .

% recovery of pure M = (118mg/132mg) ×100 = 89.39%

% impurity in the sample = (10mg/128mg of sample) = 7.8%