1. consider a crystallization of sulfanilamide in which 10ml of hot 95% ethyl al
ID: 1006757 • Letter: 1
Question
1. consider a crystallization of sulfanilamide in which 10ml of hot 95% ethyl alcohol is added to 0.10 g of impure sulfanilamide. After the solid has dissolved, the solution is cooled to room temperature and then placed in an ice- water bath. No crystals form, even after scratching with a glass rod. Explain why this crystallization failed. What would you have to do at this point to make the crystallization work?
2. Benzyl alcohol ( bp 205°C) was selected by a student to crystallize fluorenol (mp 153- 154°C) beause the solubility characteristics of this solvent are appropriate. However, this solvent is not a good choice. Explain.
3. A student performs a crystallization on an impure sample of biphenyl. The sample weigs 0.5 g and contains about 5% impurity. Based on his knowlege of solubility, the student decides to use benzene as the solvent. After crystallization, the crystals are dried and the final weight is found to be 0.02 g. Assume that all steps in the crystallization are performed correctly there are no spills, and the student lost very little solid on any glassware or in any of the transfers. Why is the recovery so low?
Explanation / Answer
1) Sulfanilamide has both polar (amine and sulfonamide) and non-polar (benzene) groups in its molecule,so it needs a solvent of intermediate polarity to solvate it properly.95% ethyl alcohol has 5% water (highly polar ) so it is a good choice.The problem in the given experiment is not with the chosen solvent pair,but with the amount of the solvent used for dissolving the solute.Minimum amount of solvent is to be used for dissolving maximum amount of solute at high temperatures for recrystallization ,which then can be extracted when the solution is cooled,else the solute won't be extracted as crystals .
2)The boiling point of Benzyl alcohol is much higher than the m.pt of fluorenol.So fluorenol will melt and evaporate by the time solvent begins to boil. Not a good choice as a solvent for fluorenol.
3) The solute,biphenyl sample here must be very well soluble in benzene at room temperature.To recover solute in the form of crystals the solute must not be soluble at room temperatures but at high temperatures so that it could be thrown out as crystals on cooling the solution.
Adding another solvent slightly polar to alter the polarity of the solvent and make biphenyl less soluble and get recovered.
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