Lab #15 – Molecular Weight by FP Depression
Name
Background
A colligative property is a property of a solution. A colligative property is a property that has a different value
for a pure solvent than for a solution made with the solvent. There are four colligative properties: vapor
pressure lowering, freezing point depression, boiling point elevation, and osmotic pressure. The spectacularly
useful thing about colligative properties is that they do NOT depend on the identity of the solvent that is added
to the solvent, but depend only on the number of solute particles added. This means colligative properties can
be used to count moles of solute even without knowing the identity of the solute. Previously we had learned
how to count moles for an unknown gas (n = PV/RT); colligative properties allow us to count moles of
unknown solid or liquid solutes, presuming we can find a suitable solvent.
In this procedure we will use the colligative property of freezing point depression. We will measure the
freezing point (aka, melting point) of a pure solvent, paradichlorobenzene (PDB). We will then add a known
mass of an unknown solute and then measure the freezing point of the resulting solution. As the name
indicates, freezing point of the PDB is depressed by the addition of the unknown solute. The freezing point
depression, ΔTf, is related to the concentration of the solution as measured in molality, m, by the following
formula:
ΔTf = kf m
ΔTf = FP depression, kf = molal FP depression constant, m = moles solute
kg solvent
You will measure the FP of the pure PDB, the FP of the PDB solution, the mass of unknown solute added, and
the mass of PDB solvent used. The molal FP depression constant is a function of the solvent; kf for PDB is
relatively large, at 7.10 ºC/m. This means PDB’s freezing point will drop significantly with the addition of
small amount of solute.
You will use the ΔTf and the kf to calculate the molality of the solution. You will then use the calculated
molality and the kg of solvent (PDB) used to calculated the moles of unknown solvent added. Moles of solute
added and grams of solute added will allow you to calculate the molar mass of the unknown. Pretty slick …
counting moles with an inexpensive thermometer.
Procedure
Start a hot water bath in a 600 mL beaker nearly full of water. Submerge a large test tube to be sure the water
won’t overflow. The freezing/melting point of PDB is in the mid-50’s, so don’t let your water bath get much
past 60ºC. If the water and PDB are overheated, it will take a loooong time for the PDB to refreeze.
Weigh a large test tube on a top-loading balance to 0.01 g and record its mass. Weigh approximately 30 g of
PDB on a piece of weighing paper, add it to the test tube, and record the mass of the test tube and PDB.
Submerge the test tube with the PDB into the hot water bath and secure it with a test tube clamp. You needn’t
wait until the water is fully heated before submerging the sample. Once the PDB is almost all melted, insert the
ASTM (loaner) thermometer into the sample and secure it to the ring stand with a thermometer clamp. Once
the PDB is all melted and in the low to mid 60’s (ºC), lower the hot water bath, dry the outside of the test tube,
and allow the PDB to air cool. As the PDB cools, stir slowly but constantly to minimize supercooling. Begin
recording the temp of the PDB when it hits about 60ºC, then record the temp every 30 seconds for 8 minutes, or
until the PDB is fully solidified.
Place the PDB back in the hot water bath to remelt it. Obtain an unknown and record its number. Use the toploading balance to pre-weigh about 2 grams of the unknown, then accurately mass the sample using the
analytical balance. Record the mass to 0.0001 g. Add the unknown sample to the remelted PDB, stirring to
mix thoroughly. Once the sample is fully melted and in the low to mid 60’s, remove the test tube from the hot
water bath and begin air cooling. Record the temp as before.
Repeat the procedure to add an additional 2 grams to the solution. Remelt, air cool, and record the temp as
before.
Prelab summary
Procedural Tips: Don’t let your hot water bath get past 65ºC.
Graph Temp vs. Time for each of the three trials, extrapolating to determine the FP.
Safety: The ASTM thermometer is very expen$ive and contains a boatload of toxic mercury. Do NOT break it.
Do NOT use the loaner ASTM thermometer to measure the temperature of the water bath.
Never use any thermometer as a stirring rod.
Disposal: Leave the solidified PDB solution in the large test tube. Use paper towel to wipe PDB off the
thermometer and stirring rod and into the test tube. Wipe the stirring rod and thermometer with a few drops of
hexane to remove trace PDB. Stack the test tubes with PDB in the fume hood.
Data and Calculations
Mass of large test tube
g
Mass of test tube plus PDB
g
Mass of PDB
g
Time (min)
0
.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
Temp (ºC)
Time (min)
0
.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
Unknown Number
Temp (ºC)
Time (min)
0
.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
Temp (ºC)
Freezing Point of pure PDB
ºC
Freezing Point of PDB with 2 g UK
ºC
ΔTf for solution #1
ºC
Freezing Point of PDB with 4 g UK
ºC
ΔTf for solution #2
ºC
Calculations Show ALL work in the space provided.
Trial #1 (PDB with 2 g UK):
SHOW WORK HERE
Calculate molality m =
Calculate moles of UK
Calculate MW of UK
moles =
MW =
Trial #2 (PDB with 4 g UK):
Calculate molality m =
Calculate moles of UK
Calculate MW of UK
moles =
MW =
Prelab Questions
A student dissolves 2.07 g of naphthalene in 27.00 g of paradichlorobenzene. The solution has a freezing point
of 48.9ºC. Using 53.0ºC as the freezing point of pure PDB, answer the following questions, showing all work.
1. What is the ΔTf?
ΔTf =
ºC
2. What is the molality, m, of the solution?
m=
m
3. How many moles of naphthalene were used?
moles =
moles
4. What is the molar mass of naphthalene?
MM =
g/mol