1. Briefly expalin why a) you shoulf not heat organic solvents over a Bunsen bur
ID: 1003787 • Letter: 1
Question
1. Briefly expalin why
a) you shoulf not heat organic solvents over a Bunsen burner flame
b) you should add activated carbon to a cool solution and then heat the mixture to boiling rather than add the carbon to a boiling solution
2. Indicate a procedure to solve the following recrystallization problems
a) oiling out
b) lack of crystal formation
c) presence of colored impurities
d) premature recrystallization in the funnel stem during gravity filtration
3. A student purified a 500 mg sample of phthalic acid by recrystallization from water. The published solubility of phthalic acid in 100 mL of water is 0.54g at 14 degree C and 18 g at 99 degree C. a) what is the smallest volumne of boiling water tthe student could use to dissoved 500 mg of phthalic acid? (show work)
Dissolution of phthalic acid in the boiling water produced a dark colored solution. The student allowed the solution to cool, added several spatulas full of activated carbon, and heated the mixture to boiling. After gravity filtration, the clear and colorless solution was allowed to cool to room temperature. Crystals formed, and the student isolated 380 mg of phthalic acid.
b) Calculate the percent recovery of phthalic acid in this experiment? show work
c) Suggest one or more procedural errors the student made that could be responsible for some loss of phthalic acid
Explanation / Answer
1
a) vaporization points of organic solvents are low and have a very low combustion temperature. This is why organic solvents may catch fire when they come in contact of fire flames. Therefore water bath or hot plate is preferred for heating organic liquids.
b) The solution may boil over and may burn skin if activated charcoal is added to the boiling solution. Therefore, activated charcoal is added after the solution is cooled to room temperature.
2
a) Oiling out occurs because of undesirable impurities or if the recrystallizing solution has a lower melting point than the boiling point of the solvent used. Oiling out prevents recrystallization. By using a solvent with a lower boiling point than the melting point of the recrystallized compound will prevent oiling out.
b) Lack of crystal formation can be solved by seeding. Here, a single crystal or a small amount of the pure crystal is added to the crystallization process.
c) Presence of colored impurities. If colored impurities are present, activated carbon can be added to the mixture.
d) To prevent premature recrystallization in the funnel stem during gravity filtration, preheating of the funnel may be performed just before the filtration process
3
a) The solubility of phthalic acid at 99 oC =18g / 100mL
Therefore, smallest volume of boiling water = (500mg x 100 mL) / 18 g
= (0.5 g x 100 mL) / 18 g = 2.77 mL
Percentage of recovery = [(380 mg) / (500 mg)] x100 = 76 %
b) Weighing the crystals before they are completely dry can cause error. Also, Using too much solvent could cause error. If impurities were not completely removed, it may become a possible source of error
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