Aya Rahav
CHEM351L (0108)
02-16-09
Post-lab Questions - TLC
Q 1. Why is it important to co-spot when identifying an unknown?
A 1. It is important to co-spot when identifying the unknown to make sure that after the TLC is
done, there is only one dot and not two close together.
Q 2. A TLC plate showed two spots with Rf values of 0.25 and 0.26. The plate was removed
from the developing chamber, the residual solvent was evaporated from the plate, and the plate
was returned to the developing chamber. What would you expect to see after the second
development was complete?
A 2. After the second development was completed, there should be a more distanced distance
between the spots. The spots moved about a quarter o the page, so on the second time, they
should move about the same (almost to the middle). In addition, as said before, the distance
between them should be about twice as it was.
Q 3. Why might it be very difficult to visualize the separation of cis- and trans- 2-butene by
TLC?
A 3. Cis and trans 2-butene have many chemical and physical properties. Although they have
double bonds between the carbon atoms, it is possible to break them and make new bonds (i.e.
temp), and for that reason, cis 2-butene can become trans 2-butene and vise versa. Since polarity
is used in TLC, it would be hard to see the difference in them.
Aya Rahav
CHEM351L (0108)
02-16-09
Pre-lab questions – Melting Points and Recrystalliztion
Q 1. Two test tubes, labeled A and B, contain substances with approximately the same melting
points. How could you prove that the test tubes contain two different compounds?
A 1. In order to determine whether the two substances are different, s mixing of both substances
and checking the mixture’s boiling point will help determine if the test tubes contain two
different compounds. If the melting point of the mixture remains the same (as previously
recorded) then the two substances are the same, but if the melting point range increases and
lowers in temperature, then the two substances are different.
Q 2. If 25 g of phthalic acid is dissolved in boiling water and then recrystallized by cooling the
solution to 14oC,
a. How many grams of phthalic acid will recrystallize out?
b. How much of the phthalic acid is unrecoverable by virtue of its insolubility at 14oC?
A 2. a. Phthalic acid is very soluble in boiling water (18g/100mL) and it is very insoluble in 14oC
(0.54g/100mL). Therefore, 139g of water are needed to dissolve all phthalic acid. Now,
calculating how many grams will recrystallize out:
18-.54=17.46
17.46*139/100=24.27
Therefore, 24.27g will recrystallize out.
b. If starting with 25g of phthalic acid and 24.27g recrystallize, 0.73g will not be recoverable.
Q 3. The goal of recrystallization is to obtain purified material with a maximum recovery.
Explain how using an unnecessarily large volume of solvent to dissolve the crude material will
impact this goal.
A 3. The yield when performing recrystallization is never 100% because while the cooling
solvent will release some crystals, some will remain dissolved in the cold solvent. So, by using
an unnecessarily large volume of solvent, more of the sample will remain in the solvent after
cooling.