Exploring Polarity & Solubility Using WebMO
The purpose of this activity is to compare the polarity of solute and solvent molecules to
determine if their polarity affects how well they mix. Solute and solvent molecules that readily
mix are said to form a solution. A common measure of how well they mix is referred to as
solubility. Solubility measures the amount of solute that can mix in a given amount of solute.
You will be using the Internet to access a program called WebMO. Using the program will
allow you to view the geometry of a molecule and determine the polarity.
During the activity students will import the following molecules and determine the polarity
direction and strength of each molecule:
C6H14 – hexane
CH3CH2CH3 -- propane
H2O – water
CH3CH2-OH – ethanol
H3C-O-CH3 – dimethyl ether
CO2 -- carbon dioxide
HF – hydrogen fluoride
Polarity is a measure of how symmetrically electrons are distributed relative to the proton
distribution in the atoms of a molecule. Molecules in which the electrons are not symmetrically
distributed are said to possess a dipole moment. The dipole moment is expressed in the unit
Debyes and can be represented by an arrow. The head of the arrow points in the direction of
greater electron concentration. The tail of the arrow points to the region of the molecule with a
lower electron concentration. You will examine the molecules to determine the direction of the
dipole arrow (polarity direction) and identify the strength of the dipole moment (calculated value
in Debyes).
To begin the activity, access the Internet and → Go to http://webmo.ncsa.uiuc.edu. Wait for the
site to redirect to the login page. Sign on with the username and password provided by your
teacher.
Exercise 1
Determine the polarity of several molecules and their solubility.
The molecules for this activity have been assembled and saved as Jmol files (*.mol) for import.
You will need to import the files one at a time and process them as indicated below in order to
observe the molecular geometries and dipole moments.
Importing the Molecules: Your instructor has saved the Jmol files in a directory to which you
have access. You will be able to access the files using the browse control during import.
Preparing to import a molecule:
This part assumes you have logged onto WebMO and are at the Job Manager" screen. Click the
“New Job” menu option at the top left of the WebMO viewing screen and select the "Create New
Job" option on the drop down menu list shown below.
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A Build Molecule" window will open as shown below.
Click on the "Import Molecule" button at the bottom right of the window. This will open the
"Import Molecule" window shown below.
Click on the "Browse" button at the center right to locate the file for import. Double click on the
file name to upload it to the import molecule window. With the file selected, click on the "Import
Molecule" button at the bottom left and the file will be imported into the "Build Molecule"
window as shown below.
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The next step is to process the molecule through the calculation engine of the software. To begin
this process, click on the blue arrow at the bottom right of the Build Molecule screen. This will
open the "Choose Computational Engine" screen shown below.
A list of five selectable engines can be seen at the center left of the window. Select the engine
titled "Mopac" by clicking on the circle to the left of the engine name. Once the engine has been
selected, click on the blue arrow at the bottom right of the window. This will open the
"Configure Mopac Job Options" window shown below.
Modify the job name by appending your initials or other identifier after the chemical formula
(C2H6O in the example). Use the pull down tab in the "Calculations” menu to view the drop
down list. Select "Geometry Optimization" as shown in the example. Finally, click on the blue
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arrow at the bottom right of the screen to submit the job. This will return you to the main Job
Manager" screen as show below.
You will see your job listed as a line entry (Number, Name, Description, Date, Status, Time, and
Actions). When the computer is finished with its computations, the job status will be shown as
"complete" (as shown in the example above). You are now ready to access the job, view the
molecule, and determine the direction and magnitude of the dipole moment. Click on the job
name to open the window shown below.
The molecule viewing window is located at the top of the screen. If you scroll down, you will
find the "Calculated Quantities" window. Locate the dipole moment listing in the calculated
quantities. Record the value given in the table provided within the lab handout. If you click on
the magnifying glass icon, the molecule in the viewing window will display a blue arrow
showing the location and direction of the dipole moment. You can rotate the molecule using your
mouse to view the dipole moment from different vantage points.
Repeat the process until you have observed all of the molecules.
Gathering solubility and experimental dipole moments for your molecules:
Information on the solubility of the molecules included in this activity can be found at a variety
of websites. To search for information of the solubilities in water or hexane, one might use a
search query such as:
ethanol solubility in water
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In a similar manner, one might search for experimentally measured dipole moments using a
query such as:
ethanol dipole moment
To completely fill in the chart below, you will need to do a search on the Internet for the
experimental dipole moments and solubilities of each of the molecules in water and hexane.
Record the dipole moments obtained from WebMO (calculated) and the experimental dipole
moments from the Internet search. Use parentheses to set off the experimental value. The
polarity of a molecule increases as its dipole moment increases.
Formula
Name
Dipole
(Debye)
Polarity
C6H14
H2O
Solubility Predictions
H2O
C6H14
xxxxxxxxxx
xxxxxxxxxxx
CH3OCH3
CH3CH2OH
HF
CH3CH2CH3
CO2
A completely nonpolar molecule is expected to have a dipole moment at zero. The dipole
moment is a measure of the force of attraction or repulsion per unit of mass on the affected
particle. The positive end of a dipole attracts the negative end of other dipoles; it repels the
positive end of other dipoles. Hence, polar molecules tend to exhibit mutual attraction for one
another. They interact only very weakly with nonpolar molecules. From this tendency we get the
simple rule “Like dissolves like”. Using this rule, predict which of the molecules in this activity
are soluble in H2O and/or C6H14. Compare your predictions with the solubilities found on the
Internet.
Dipole Moment
Order
Least
To
Greatest
Formula
Solubility (include units)
H2O
C6H14
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Questions:
1) What relationship do you see between the polarity of a substance and its solubility in
water? (In the first six molecules.) What reasons can you give for this relationship.
(Hint: think about what is happening in a polar molecule vs. a nonpolar molecule.)
2) Rank the substances in order of their polarity from least polar to most polar. Does the
solubility of these molecules in water show a pattern? If you observe a general pattern, or
relationship, between polarity and water solubility, describe the pattern.
3) Does the solubility and polarity of CO2 fit the pattern? If not, what possible explanation
can you offer?
4) Assuming you are able to locate the solubilities of the molecules in hexane, does the
solubility of the molecules in hexane show a pattern? If you observe a general pattern, or
relationship, between polarity and hexane solubility, describe the pattern.
5) In what ways has this activity helped you understand the relationship between solubility
and polarity?
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Teacher Notes:
Determining Solubility:
Keeping in mind that “Like dissolves like”, which of the following are soluble in H2O and/or
CCl4.
Formula
Name
C6H12
hexane
H2O
water
CH3OCH3
dimethyl ether
CH3CH2OH
ethanol
HF
Hydrogen fluoride
CH3CH2CH3
propane
CO2
Carbon dioxide
Dipole
(Debye)
0.000
Polarity
Soluble in H2O
Soluble in C6H12
nonpolar
1.739
(1.85)
1.254
(1.30)
1.553
(1.69)
1.404
(1.91)
0.005
(0.08)
0.000
polar
0.013g/L
@ xxxxxxxxxx
20ºC
xxxxxxxxxxx
polar
71g/L @ 20ºC
polar
miscible
polar
miscible
nonpolar
0.07g/L @ 20ºC
nonpolar
1.45 g/L @ 25ºC