1. By marking yes (Y) or no (N) in the space provided, specify which of the foll

Question

1. By marking yes (Y) or no (N) in the space provided, specify which of the following criteria are met by a good solvent for a recrystallization.
_______a. The solutes are soluble in the cold solvent.
_______b. The solvent does not react chemically with the solutes.
_______c. The solvent is polar rather than nonpolar.
_______d. The boiling point of the solvent is above 100 °C (760 torr).
_______e. The boiling point of the solvent preferably is below the melting point of the solute.
2. Give the criterion applied in this experimental procedure to classify a solute as being “soluble” in a particular solvent.
3. Review the functional groups present in resorcinol, benzoic acid, naphthalene, and acetanilide. Predict
whether these molecules are expected to be polar (P) or nonpolar (NP).
Resorcinol ______ Benzoic acid ______ Naphthalene ______ Acetanilide ______
4. Why is it important to
a. avoid inhaling vapors of organic solvents?
b. know the location and operating instructions of the nearest fire extinguisher when using 95% ethanol or petroleum ether for testing solubilities?
NAME: ____________________________ DATE: ___________ TA: ______________________
5. The flash points of 95% ethanol and petroleum ether (bp 60–80 oC) are __________ and ___________, respectively.
6. By marking gravity (G) or vacuum (V) in the space provided, indicate which of the two different filtering
techniques is more suitable for each of the following operations.
______ a. Hot filtration.
______ b. Removing decolorizing carbon.
______ c. Isolating recrystallized solute from solution.
7. Why is flameless heating used for heating a solution in hexane or diethyl ether during a recrystallization?
8. What is the process of seeding, as it applies to recrystallization? What purpose does it serve?
9. What is meant by the term, “oiling out,” as it applies to crystallizations?
6. How is the purity of a recrystallized solid assessed?

Explanation / Answer

1. By marking yes (Y) or no (N) in the space provided, specify which of the following criteria are met by a good solvent for a recrystallization.
___N___a. The solutes are soluble in the cold solvent.

Explanation: We want the solutes to be soluble in the hot solvent (at high temperatures) because then we can cool down the solvent, causing the solute to crystallize.

___Y___b. The solvent does not react chemically with the solutes.

Explanation: This is because we want to isolate the crystals of the solute and not a new compound that would be formed after the reaction between solvent and solute.
___N___c. The solvent is polar rather than nonpolar.

Explanation: This is not always true, sometimes a polar solvent will be better and sometimes a nonpolar solvent will be better, this will depend on the nature of the solute (if the solute is ionic or polar we may want to use a polar solvent, if the solute is nonpolar we may want to use a nonpolar solvent).
___N___d. The boiling point of the solvent is above 100 °C (760 torr).

Explanation: A good crystallization solvent has to be volatile so it must have a low boiling point, so once the crystals are formed we can easily separate the solvent by evaporation.
___Y___e. The boiling point of the solvent preferably is below the melting point of the solute.

Explanation: This is because if we separate the crystals and the solvent by evaporation, we do not want the crystals to melt but the solvent to evaporate first.

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