(CHE 276) Organic Chemistry Lab Fall 2011
Experiment 2: Purification of a Solid by Recrystallization
Appendix C: Questions
Answers to questions should be typed and submitted in Appendix C of your Lab Report. Hand written answers will not be graded. Supporting figures, if any, may be neatly hand drawn.
1. When a solution is allowed to crystallize slowly, impurities are excluded from the growing crystal structure because:
a. the molecules in the crystal lattice are cooler than the solvent and impurities would have
too high a melting point to join the lattice
b. the molecules in the crystal lattice are in equilibrium with the molecules in solution and molecules that don't fit well into the lattice (impurities) are likely to return to the solution
c. impurities could never join the lattice because they are always liquid at room temperature
d. during slow crystallization, impurities are prevented from joining the crystal structure
because of their ionic strength
2. An impure mixture contains 10.0g of Compound A, and 0.20g of Compound B. The solubilities of these two compounds in water at two different temperatures are given below.
solubility at 0°C(g/10mL) / solubility at 100°C
(g/10mL)
Compound A / 0.035 / 0.690
Compound B / 0.29 / 5.40
a. What volume of boiling water (to the nearest mL) is needed to dissolve the 10g of impure
compound A? Show your work.
b. How much compound A will crystallize after cooling to 0°C? Show your work.
3. You are given a sample that is slightly impure. When you determine the melting point, it melts at 51-55 deg. C. You are told that the compound is either benzophenone (mp 49-51 deg. C), maleic anhydride (mp 54-56 deg. C), 2-nitroaniline (mp 71-73 deg. C), or naphthalene (mp 80-82 deg. C). Which do you think it is? Explain your reasoning.
a. benzophenone
b. maleic anhydride
c. naphthalene
d. 2-nitroaniline
e. It cannot possibly be any of these choices!