Consider a crystallization of sulfanilamide in which 10 mL of hot 95% ethyl alco

Question

Consider a crystallization of sulfanilamide in which 10 mL 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? Assume that starting over again with a new sample is not an option. (You may need to refer to Technique 11, Figure 11.2.)

Explanation / Answer

The solubility of sulfanilamide in boiling 95% pure ethanol (bp of ethanol is 78C) is 210 mg/mL.

We had 0.10 g impure sulfanilamide and we dissolved it in 10 mL hot ethanol.

The actual volume of boiling ethanol that should have been used is (0.10 g)*(1000 mg/1g)*(1 mL/210 mg) = 0.476 mL 0.48 mL.

The reason no crystals were formed is because we used excess hot ethanol (almost 20 times higher volume than required) and thus all the sulfanilamide dissolved in the ethanol.

In order to make this crystallization work again, what is required is to carefully heat the mixture till 78C to evaporate the excess ethanol. The melting point of sulfanilamide is 165C and hence the heating shouldn’t cause any change to sulfanilamide. Once the excess solvent has been evaporated, the mixture should be allowed to cool slowly to room temperature when crystals of pure sulfanilamide will be obtained.

Related Questions

Navigate

• Browse (All)
• Browse C
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.