Chromatography Quiz
Introduction to Forensic Science
November 10, 2015
Name:
1.
Directions: Fill in the following chart, using the attached periodic table, which shows the
electronegativity of the elements in each pair. (8 points)
If the calculated electronegative difference is
ΔEN > 1.7 then an ionic bond forms
0.6 < ΔEN < 1.7 then a polar covalent bond forms
ΔEN < 0.6 then a non-polar covalent bond forms
Elements
Electronegativity
Element 1
Element 2
Electronegativity
Difference
C to H
H to O
C to Cl
N to H
C to O
K to F
Na to Cl
H to H
The molecule below is acetone.
The molecule below is hexane.
Bond Type
2.
The polarity of a molecule is determined by evaluating the polarity of the bonds
between atoms in that molecule. Using your results from the table above and looking at the
bonds present in the two different molecules, explain why acetone would be more polar than
hexane? Also, explain why acetone would have a higher affinity for polar molecules than
hexane. (8 points)
In the TLC lab, ink from the t-shirts was spotted on silica-coated plates. The plates were then
placed in a beaker that had a small amount of ethyl acetate in it.
Circle the letter of the best answer below. (2 points each)
3.
The stationary phase in the TLC lab was
a. the blue ink
b. the silica coating
c. the ethyl acetate
d. the pyrex beaker
4.
The mobile phase in the TLC lab was
a. the blue ink
b. the silica coating
c. the ethyl acetate
d. the pyrex beaker
5.
An ink sample that travels farther up the TLC plate has a greater attraction to
a. the mobile phase
b. the stationary phase
c. the pyrex beaker
d. the plastic in the TLC plate
6.
The molecule below is ethanol.
Examine the molecule and determine the polarities of the bonds between the atoms in the
molecule. (Again, the table that you completed above will be useful.) Label each bond in the
ethanol molecule with a “P” (for polar bond) or “NP” (for non-polar bond). Using what you
know about the nature of the bonds in the ethanol molecule, explain why ethanol can be mixed
with either water (a highly polar substance) or gasoline (a non-polar substance) and these
mixtures will not separate. (8 points)
7. The image below shows a sample being separated using chromatography. Calculate the Rf of
the green (at 2.7 cm) and yellow (at 1.8 cm) parts of the red (at 0.0 cm) sample. (5 points)
Green Rf:
Yellow Rf:
Why did the two parts of the red sample separate as they moved up the chromatography
plate?