DETERMINING MOLAR MASS BY FREEZING POINT DEPRESSION
Purpose: To determine the molar mass of a solute by freezing point depression.
Intro: The solvent in this lab is naphthalene, C10H8, which has the following structure and is nonpolar.
The freezing point for naphthalene is 80.2°C and it Kf = 6.9 °C/m. The solute in this lab is sulfur which can exist in two
different forms, both of which act nonpolar. Thus the sulfur can dissolve into the naphthalene when the naphthalene is
melted.
To find the molar mass of the solute, you will use the freezing point depression equation, ΔT f = Kfmi. In this case, i is one
since both forms of sulfur dissolve as one solute particle per unit particle. Taking the equation for ΔT f and solving for
molar mass (which is in the molality part of the equation), the resulting equation is:
solute
g solute
ΔTf kg solvent
f
Once the molar mass is determined, you will determine the molecular formula for the sulfur by dividing the experimental
molar mass by 32.07 g (molar mass of S).
The freezing point curve for a pure substance may look like the diagram to the right. The freezing
point will be the temperature where the graph plateaus since the kinetic energy doesn’t change
during the physical state change. The little dip before the horizontal portion of the graph is when
supercoiling may occur. For a solution, the graph will look a little different. Instead of a long
horizontal portion, the graph will start gradually curving downward shortly after leveling off. This
occurs because as the solution freezes, the part of the solution that hasn’t frozen yet will become
more concentrated since the solute remains in the solution. There will still be a somewhat
horizontal portion of the graph to determine the freezing point of the solution. The procedure will
give you a method of determining the freezing point of the solution when this happens.
Equipment:
 Logger Pro temperature probe and interface
 XL test tube
 wire loop
 600 mL beaker
 burner
CAUTION: Do not expose the naphthalene to an open flame—naphthalene is flammable.
Procedure:
1) Clean and dry an XL test tube. Weigh it to the nearest 0.01 g.
2) Add approximately 10 g of naphthalene to the test tube.
3) Add approximately 0.5 g of sulfur to the naphthalene. Weigh the test tube to the nearest 0.01 g. Record this
mass.
4) Set up the collection time on the computer for 1000 seconds. Go to “Experiment” tag and then “Data Collection”
and change the time length.
5) Melt the solid in a hot water bath. When able, insert the wire loop into the test tube and stir until all the sulfur has
dissolved.
6) Insert the temperature probe and after the temperature stabilizes, begin the collection process on the computer.
Remove the test tube from the water bath. Stir with the wire loop until you no longer able to. Continue to collect
data for a minute after you are sure the liquid has frozen.
7) Autoscale your graph. Determine where the freezing plateau is located and linear fit it. Then linear fit the
downward slope at the beginning of the curve. Then go to “Analyze” and select “Interpolate”. ove the
interpolation line until the point on both lines overlap to give you the closest match and then determine the
average of the two readings. Record this temperature. Double click to get off the graph and print off a copy of the
graph (only the graph) for all the partners.
8) Reheat the mixture to remove the probe and wire. Dump the mixture into the waste container. Then rinse out the
test tube and wipe the probe and wire with acetone to remove any of the mixture. Finally wash the test tube well.
Throw the wire away if after the second trial.
9) Obtain another clean test tube and repeat steps 1-8.
LAB REPORT SETUP
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title
purpose
data:
freezing point of naphthalene: ________
Kf of naphthalene: ________
DATA TABLE
mass of empty test tube
mass of test tube + naphthalene
mass test tube + naphthalene + sulfur
freezing point of solution (from graph)
ΔTf *
mass of naphthalene*
mass of sulfur*
molar mass of sulfur*
molecular formula of sulfur*
*calculations needed
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calculations for each indicated step for the first trial.
questions:
1) Define the term “colligative property”.
2) For a given number of moles of solute, why do ionic substances have a larger effect on the freezing and boiling
points of solvents than do nonionic substances?
3) Explain why the temperature doesn’t level off with the solution as it would with a pure substance.
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conclusion: Discuss the process of lowering the freezing point from that of pure naphthalene when having a
solution. Explain the process followed by the calculations. Explain any possible errors not “I measured wrong”
and or any such lame excuses). Tell your final answer.
BE SURE TO ATTACH YOUR GRAPHS TO THE REPORT.