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MOLECULAR WEIGHT
I. Rationale
The molecular weight of a substance is one of its most important characteristics. If the
composition of a substance is known, the molecular weight is often the key necessary to
determine its structure.
One of the methods commonly used to determine the molecular weight of a substance is
based on the observation that the freezing point of a solution changes in proportion to
the number of molecules of the substance which is dissolved in the solution. In this
exercise we will make use of this fact to determine the molecular weight of camphor.
II. Prerequisites
1. Make sure that you know how to graph scientific data.
2. Bring a pencil so that you can draw graphs.
3. As required In GENERAL INFORMATION - LAB, be ready to turn in a COPY of your
Equipment list
4. Be able to answer the following questions:
A. What is meant by “molal freezing point depression”?
B. What is the molecular weight of camphor? (Look it up.)
C. What should be the range on the vertical axis of your graph?
III. Objectives
When you are finished with this laboratory exercise you
should be able to:
1. Measure the freezing point of a liquid and determine the decrease in freezing point
which occurs when a solute is dissolved in the liquid.
2. Prepare a graph of temperature of the liquid vs. time, and use this graph to determine
the freezing point of the liquid.
3. Calculate the molecular weight of a solute based on the freezing point depression
which is produced in a solvent.
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IV. Lab Exercise
1. Introduction
Whenever one mole of a covalent solute is dissolved in 1000 gm of benzene, the
freezing point of the solution is 5.12o lower than the freezing point of pure benzene. The
decrease in freezing point is proportional to the number of moles of solute dissolved: 0.5
moles causes a decrease of 2.56o, or half of that caused by one mole. The decrease in
freezing point of 1000 gm of solvent caused by one mole of any solute is characteristic of
each solvent, and is called the molal freezing point depression, molal Îfp. The molal Îfp
for benzene is therefore 5.12oC; the molal Îfp for naphthalene is 6.9oC.
Therefore, if you measure the decrease in freezing point that occurs when a known
weight of a solute is dissolved in 1000 gm of a specific solvent, you can calculate the
number of moles of the solute that was dissolved, and from that the molecular weight of
the solute can be calculated.
2. The freezing point data
Thoroughly clean and dry a large test tube and
thermometer. Warm the test tube over a flame
to ensure drying. Usually, when we weigh a
solid, we use a weighing pan. But any
container could be used. In this case we will
use a test tube as the container instead of a
weighing pan.
Weigh the test tube to the nearest 0.01 gm.
test tube
_____gm
Fill half-full with naphthalene and determine
the mass of the naphthalene to the nearest
0.01 gm by difference.
test tube + naphthalene
_____gm
naphthalene
_____gm
Figure 1
Assemble the apparatus as shown in Figure 1:
Make sure to use a 250 ml beaker full of water. Place the cardboard cap (a 3 cm x 3 cm
square with a hole punched in the center) as shown, but do not insert the thermometer
until the solid below the waterline in the beaker has melted. Place the test tube as close
to the bottom of the beaker as you can without actually touching the bottom.
Heat the water until the naphthalene is completely melted.
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Remove the flame but leave the test tube in the hot water.
When the temperature drops down to 85o begin stirring gently with the thermometer and
begin to record the temperature to the nearest half-degree every 30 seconds while
continuing to stir until the temperature has dropped to 70oC.
NOTE: You may remove the thermometer at this point -- but DO NOT clean it and DO
NOT lose any of the solid which sticks to it!
Refill the beaker to replace water which has boiled away.
Add about 0.7 gm of camphor (weighed to the nearest 0.01 gm), being sure that no
camphor is lost on the side of the test tube.
camphor
_____gm
Melt the naphthalene again by reheating the water.
Stir gently with your thermometer until the camphor is dissolved.
Stop heating and after the temperature drops to 85o begin stirring again. Record the
temperature to the nearest half-degree every 30 seconds until the temperature reaches
70oC.
You will probably see little, if any, period of constant temperature like you saw with pure
naphthalene. Instead, the temperature will continue to decrease as the solution freezes.
Remove the thermometer from the test tube and wipe it clean with a paper towel before it
cools much more.
Remove the test tube from the hot water and allow it to cool to room temperature. Then
discard it in the bottle in the hood labeled Organic according to the directions given in the
box at the end of this exercise.
3. Determination of freezing points
Prepare a single graph (as illustrated in Figure 2) for both your data for pure naphthalene
and for the camphor in naphthalene. Plot temperature on the vertical axis vs time on the
horizontal axis. NOTE: Use a range of 70o to 85o on the vertical axis.
Draw the best line among the points for each set of data. Label each curve with the
name of the solvent or the name of the components of the mixture.
Make sure that the overall graph is labeled properly. Your graph should resemble
Figure 2.
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Figure 2
Notice that each "cooling curve" is made up of two more or less straight lines. The slope
of the lines starts off rather steeply, then at a certain temperature you will notice that the
slope becomes less steep - it is not cooling at all (the pure solvent), or it is cooling more
slowly than before (the mixture). For each curve, the temperature at which the curve
changes slope is the freezing point.
Record the freezing points which you can see from your curves:
freezing point of pure naphthalene
_____
freezing point of naphthalene + camphor _____
4. Molecular Weight
Calculate the difference in freezing point from your two freezing points:
ÎT _____oC
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Use the following equation, your experimental ÎT, and the molal Îfp for naphthalene
(given in the introduction), calculate the number of moles of camphor per 1000 gm of
naphthalene (Mc):
Mc = __ÎT____
molal Îfp
= __________ moles camphor per 1000 gm naphthalene
Calculate the weight of camphor per 1000 gm of naphthalene (Wc), based on the
weights that you used:
Wc = gm camphor used x 1000 = _____ gm camphor per 1000 gm naphthalene
gm naphthalene used
Calculate the molecular weight (the weight of one mole) of camphor from the above two
results:
molecular weight = Wc = __________
Mc
How does this compare with the molecular weight which you looked up?
Waste Disposal:
Carefully place your test tube and its contents into the "Organic" container. Tilt the
waste bottle and let the test tube slide down the side to avoid splashing.