Name: _________________________________________
Date:____________________________
Optional Homework !!!! Do not turn in!!
More IMF problems since some of you need them!
1.) Why are dipole-dipole interactions weaker than ion-dipole interactions?
Dipole-dipole attractions are weaker than ion-dipole attractions because species with
dipoles only have partial charges. A full charge on an ion is able to attract partial charges
to itself more strongly than partial charges are able to attract other partial charges. If the
attraction is stronger, then the molecules/atoms/ions are held together more strongly.
2.) Two molecules – one polar the other nonpolar – have similar molecular masses. Which
one is more likely to have the higher boiling point? WHY?
The polar species should have the higher boiling point. If they have similar molar
masses, then the strength of the London forces between the species will be similar. BUT,
if a species is polar, it will have dipole-dipole IMFs in addition to the London forces. The
polar species has an “extra” attraction holding the molecules together so more energy (in
the form of heat) would need to be added for the molecules to “let go” of one another and
boil away.
3.) Why do the strengths of London (dispersion) forces generally increase with increasing
molecular size?
Generally speaking, as size increases, so does the surface area of the molecule. The larger
the surface area, the more points of contact there are between molecules and therefore, the
molecules are more attracted to one another and more “stuck” together.
4.) Which IMFs are present in each of the following molecules. List them all and indicate
which is the strongest IMF: (Hint: Lewis structures might help!!!!)
a. H3PO4
b. SO2
c. MgCl2
O
H
O
P
O
H
O
H
O
S
O Cl
Mg
Cl
H3PO4
this species has H-bonding (it has O-H bonds and l.p. electrons on the
oxygens. It is polar (the shape is tetrahedral and the P is not bonded to all the same
thing (O-H are different than O) So this substance has London, dipole-dipole, and Hbonding. H-bonding is probably the strongest IMF in this molecule since there are 3
O-H bonds and each O-H can H-bond 3 times!
SO2 this species is polar (it has a bent shape, it has a l.p. of electrons on the central
atom!). This species has London and dipole-dipole. Dipole-dipole is the strongest
IMF in this molecule.
MgCl2 this species is ionic. It has ion-ion and London forces. Ion-ion forces are the
strongest in this species.
1.) What is the freezing point of an aqueous sucrose solution that has 25.0 grams of
C12H22O11 per 100.0 grams of water? (the freezing point of pure water is 0.00oC, the Kf for
pure water is 1.86oC/m)
Tf = Kf m i
(i for sucrose = 1)
molality =
Tf = 1.86oC (0.730 m)
Tf = 1.36oC
moles sucrose
0.0730 mole
=
= 0.730 m
kg water
0.1000 kg
C: 12 x 12.01 = 144.1
H: 22 x 1.01 = 22.2
O: 11 x 16.00 = 176.0
mm = 342.3 grams/mol
New freezing point:
25.0 g x
1 mole
= 0.0730 moles
342.3 grams
-1.36oC
2.) An aqueous solution is prepared by dissolving 1.50 grams of hemocyanin in 0.250 L of
water. The solution has an osmotic pressure of 0.00342 atm at 277K. What is the molar
mass of hemocyanin? Assuming that the molality and molarity are the same since the
solution is so dilute, what is the freezing point of the solution?
 = MRT(i) - where i = 1 since hemocyanin is a covalent molecule
0.00342 atm = M (8.206 x 10-2 Latm/molK)(277 K)
0.00342 atm = M (22.73 Latm/mol)
0.00342 atm
22.73 Latm
mol
= M = 0.000150 mole/L
But we don’t have 1 L we only used 0.250 L! 0.000150
mole
x 0.250 L  3.76 x 10-5 moles
L
hemocyanin
mm = grams/moles = 1.50 grams/3.76 x 10-5 moles =
4.00 x 104 grams/mole (40,000 grams/mole)
since molarity = molality them 0.000150 mole/L = 0.000150 moles/kg
Tf = Kf mi (where i = 1)
Tf = 1.86 oC/m (0.000150 m)
Tf = 0.000279 oC
the new freezing point of the solution is -0.000279oC
3.) Define isotonic solution:
The concentration inside and outside the cell is the same. Water is free to move with no
consequence to a cell.
4.) Define hypertonic solution:
The concentration of solute inside the cell is less than the concentration of solute outside
the cell. This will make the water inside the cell flow out of the cell. The cell will shrink
or become dehydrated as the water leaves (Inside the cell is the high water concentration,
outside the cell is the lower water concentration and since it is only water that can cross,
the water will flow from the region of high concentration to lower concentration – thus
the water leaves the cell)
5.) Define hypotonic solution:
The concentration of solute inside the cell is lower than the concentration of solute
outside the cell. This means that the concentration of the water outside the cell is greater
than inside the cell. The water will flow from a region of high concentration to low
concentration. In this case, the water will enter the cell. If the cell takes on too much
water it will lyse, or burst.