Chem 238 Exam #I
Name
February 7", 2006
sopts
1. Listed below are the solubility-temperature data for an organic substance A dissolved in
water.
a.) (5pts) How much water would be required to dissolve 1.5g of substance A at 80°C?
b.) (5pts) How much of substance A would remain dissolved in the solution after the mixture
was cooled to 20°C?
3 m ~ ( x =) 0.3,
1OOmL
remain
2. (6pts) What is the difference between precipitation and crystallization?
Precipitation is fast crystal formation, which trap the impurities.
Crystallization is slow formation of the crystals. The impurities are not allowed to be
trapped within the crystal lattice, and will stay dissolved in the solution.
3. (4pts) Why is it important to cool the solution slowly during crystallization?
You want to crystallize slowly to allow the intermolecular forces of the molecule to align
and match up. This will exclude the impurities from the crystal lattice.
4. (15pts) Which of the following pairs will be miscible/soluble with each other? Please circle
your answers.
CH2CI2
/
/
AND
5. (6pts) Why is benzophenone soluble in both methanol and hexane? Explain your answer in
terms of polarity and intermolecular forces. Draw the interaction between benzophenone and
methanol in solution.
Benzophenoue has two non-polar benzene rings, which allows it to be soluble in non-polar
hexane. Benzopheno~lealso has a polar carboilyl group which is able to hydrogen bond
with slightly polar methanol.
+
c l - 0 ,
6. Refer to the structure below for parts a-c.
You dissolve 0.060g of compound C in 2ml of methylene chloride (CH2C12). Next you add 4ml
of 1M HC1 and shake. After the layers have separated you pull the methylene chloride layer out
and evaporate off the solvent. The test tube contains 0.017g of compound C.
a,) (Spts) What is the distribution coefficient (K) for compound C between methylene chloride
and 1M HCl? You may assume that the concentration term for methylene chloride is in the
denominator.
60mg - 17mg = 43mg left in HC1 layer
bl) (4pts) Draw the structure of compound C that is dissolved in 1M HC1.
c.) (4pts) Would you expect the distribution coefficient (K) for compound C in methylene
chloride and water to be smaller, larger, or the same as your answer in 8a? Briefly explain your
answer.
K mould be smaller. Since compoui~dC is mostly non-polar it will be more soluble
in methylene cl~loridethan in water.
7. Shown below are the solubility versus temperature graphs for malonic acid (left) and
phthalic acid (right). The solubility behavior in three different solvents is shown.
ethanol
Solubility
(g/mL)
Malonic acid
Phthalic acid
a,) (4pts) List the solvents that you could use to recrystallize malonic acid.
Ethanol, diethyl ether
b.) (4pts) For which solvent for phthalic acid would you say that it is "soluble hot and insoluble
cold"?
Hz0
8. (6pts) You have an organic compound that has a distribution coefficient (K) of 15. You
dissolve 50mg in 5rnL of water. Would it better to extract the water layer once with 3mL of
ether or twice with 1 . 5 d each of ether? Please support your answer with calculations. Assume
the concentration of ether is in the numerator.
lStextraction with 1.5mL ether:
50-x
~ = 1 5 = -C2
= - ether
C, water
~ = 1 15 5 =m ~~ 5 0=- x - X -5=
x1.5
x
5mL
250 - 5 x = 2 2 . 5 ~
250-5x
1.5~
x = 9.lmg in water, 40.9mg in ether
2"d extraction with 1.5m1, ether:
9.1-x
K = 1 5 = - =C2
C,
15 =
45.5 - 5 x
1 CY
ether
water
45.5 - 5x = 2 2 . 5 ~
Total = 40.9mg + 7.5rng = 48.4mg in ether
x = 1.7mg in water, 7.5mg in ether
Extraction with 3mL ether:
50-x
~ = 1 5 = - =C-,
C,
etlzer
water
250 - 5x = 45x
x = 5.Omg in water, 45.0mg in ether
You are only able to extract 45.0mg of the compound with 1 extraction of 3mL versus
48.4mg in 2 extractions with 1.5mL each. It's better to perform 2 smaller extractions, than
one large one.
9. (12pts) You are given a mixture of: A, Byand C below. Show how you would isolate and
purify each of these compounds. Draw a flow chart, which includes the solvents and the
structures of the compounds in each layer of the separation.
I
Add lMNaOH
ether layer
I
ether layer
aqueous layer
Add 1MHC1
aqueous layer