Exercise 15 Page 1
Illinois Central College
CHEMISTRY 130
Name:___________________________
Molecular Mass Determination by Boiling Point Elevation
Objectives.
In this experiment we will use the fact that the presence of a nonvolatile solute in a solution causes its
boiling point to rise proportionate with its molality. By measuring the amount of boiling point elevation,
we can determine the molecular mass of the solute.
Background
The colligative properties of solutions are those properties that depend on solute concentration.
These properties include:
boiling point elevation
freezing point depression
osmotic pressure
vapor pressure reduction
Each of these properties is directly proportional to solute concentration. In today's lab we will look at
the effect on boiling point due to the presence of a nonvolatile, nonelectrolyte solute.
The relationship between solute concentration and boiling point elevation is defined by the following
expression.
∆T = i k b m (1)
where "i" is defined as the van't
Hoff Factor (the number of particles a solute dissociates into in solution...which for a nonelectrolyte,
is 1), "k b" is the boiling point elevation constant characteristic of the solvent being used (for water, kb is
0.512oC/m) and "m" is the molality of the solute concentration (that is, moles of solute per kilogram of
solvent).
Since molality is defined as m =
moles of solute
kilograms of solvent
, we can substitute this expression into the equation
(1) above to obtain:
moles of solute
∆T = i k b ( kilograms of solvent )
(2)
We can further expand this equation by using the fact that moles of solute is defined as:
moles of solute =
grams of solute
molecular mass of solute
Exercise 15 Page 2
By substituting this expression into equation (2) we obtain the following equation:
grams ofsolute
( molecular mass ofsolute )
∆T = i k b ( kilograms of solvent )
(3)
If we solve equation (3) for Molecular Mass we obtain:
Molecular Mass=
i (k b ) (grams of solute)
(kilograms of solvent) (∆T)
A careful examination of this equation reveals that we simply dissolve a measured quantity of solute (in
grams) into a measured quantity of solvent (in kilograms) and measure the change in boiling point in
order to determine the Molecular Mass of the unknown solute.
Procedure
1. Connect the Go-LinK to the USB connestor of
your computer and connect the Stainless Steel
Temperature Probe to the Go-LinK.
2. Start the Logger Pro program on your computer.
Open the file “Boiling Point Elevation” from the
Chemistry with Computers folder. You should
now see the screen shown here. This file allows you
to collect temperature readings at half-second
intervals and display them graphically.
Thermistor Calibration
CHECK WITH YOUR INSTRUCTOR TO SEE IF CALIBRATION IS NECESSARY
In most cases, the Stainless Steel Temperature Probe will never need to be calibrated. However, if
the need arises the sensor can be custom calibrated. You will need a good thermometer and three
containers of water at three different temperatures.
1. Open Logger Pro and from the Menu Bar choose Experiment/Calibrate and then choose the
Temperature Probe .Choose Calibrate Now. For each of the three water baths, place the
temperature probe in the bath with the thermometer.
2. Allow both readings to stabilize, click
thermometer.
, and enter the temperature reading measured by the
3. Click Done after the third water bath reading.
Exercise 15 Page 3
Note: If you want the calibration to be saved with the file for later use after you calibrate, you
need to do two things:
Go to the calibration dialogue box and choose Calibration Storage from the pull down
menu. Then set the Calibration Retrieval Preference to Experiment file.
Save the Experiment file.
Running the Experiment
To obtain a baseline boiling point, we will measure the boiling point of pure water first, then introduce a
measured amount of ethylene glycol (a nonelectrolyte) and measure the new 'elevated' boiling point. In
order to illustrate the effect of the van't Hoff Factor, we will also measure the elevated boiling point of a
NaCl solution of identical molality (which ionizes into twice as many solute particles as the
nonelectrolyte, ethylene glycol).
The Baseline Boiling Point
Temp Probe
1. Obtain a 250 mL round-bottom florence flask. Using a graduated
cylinder, measure 100 mL of distilled water (Density=1.00 g/mL)
into the flask. Place 3 or 4 boiling chips in the flask to ensure
smooth boiling. Fit the flask with the Stainless Steel
Temperature Probe , adjusting the height of the probe such that
the tip is approximately half-way to the bottom. (See Figure 1.)
2.
3.
Turn the hotplate temperature to the maimum and watch the
temperature displayed on the screen. When the temperature
approaches 70oC, click the Collect button
on the Logger
Pro Tool Bar.
The program will wait for a triggering
temperature of 80 oC before it begins plotting.
Once the temperature reaches 80 oC, allow
the program to collect data until a boiling
"plateau" has been well established as in the
screen shown here. At this point you can click
the Stop button
on the Tool Bar.
Fig. 1
Exercise 15 Page 4
Elevated Boiling Point
1. When the first plot has been completed, empty the flask and obtain a fresh 100 mL of distilled
water rather then waiting for it to cool.
2. To the water in the flask, add 22.5 mL ethylene glycol (Density=1.10 g/mL) and 3 or 4 boiling
chips. Mix well and begin heating. As before, when the temperature approaches 70oC, click the
Collect button
on the Tool Bar. You will be prompted to choose how to handle the
previous data collection. Choose Store Latest
Run.
3. As before, the program will begin plotting points
when the temperature reaches 80oC. When the plot
has established a good boiling "plateau" click the
Stop button
on the Logger Pro Tool Bar.
You should now see both plots on the screen as
shown here.
Abnormal Boiling Point Elevation
1. When the second plot is complete, again obtain a fresh 100 mL sample of water and introduce
23.5 grams NaCl and 3 or 4 boiling chips. This is sufficient NaCl to produce the same molality as
was used in the ethylene glycol determination. The difference here, however, is that NaCl is an
electrolyte with a van't Hoff Factor of approximately '2'. We should see a significantly larger
(almost twice as great) boiling point elevation in this case.
2. Mix the NaCl until it is completely dissolved, and begin heating. As before, when the temperature
approaches 70 oC, click the Collect button
on the Tool Bar. You will be prompted to
choose how to handle the previous data collection.
Choose Store Latest Run.
3. As before, the program will begin plotting points
when the temperature reaches 80oC. When the
plot has established a good boiling "plateau" click
the Stop button
on the Logger Pro Tool
Bar. You should now see all three plots on the
screen as shown here.
Exercise 15 Page 5
Data Analysis
1. Scroll through the data for your three experiments and estimate the boiling temperature of the
three solutions to the nearest tenth of a degree. Record the boiling point of the pure water
and the boiling point of the water/ethylene glycol mixture on your Report Sheet.
2. Determine the change in boiling point (∆T) between the pure water and the water/ethylene glycol
mixture. Use this ∆T to determine the Molecular Mass of ethylene glycol.
3. Select File/PrintGraph and print 2 copies of this plot to be attached to your lab reports.
4. No calculation is required with the water/NaCl plot. It is displayed merely to demonstrate the
effect of the van't Hoff Factor in boiling point elevations. After you have printed your graphs, exit
the program.
Exercise 15 Page 6
Exercise 15 Page 7
Illinois Central College
CHEMISTRY 130
Name:___________________________
REPORT SHEET
Molecular Mass Determination by Boiling Point Elevation
Ethylene Glycol Determination
Mass of water used (grams)
Boiling point of pure water (oC)
Volume of ethylene glycol used (mL)
Mass of ethylene glycol used (grams)
Boiling point of ethylene glycol/water mixture (oC)
(∆T) , Boiling point elevation (oC)
Molecular mass of ethylene glycol (g/mol)
Calculations:
Abnormal Boiling Point Elevation
Mass of water used (grams)
Mass of salt used (grams)
Molality of salt solution
Boiling point of salt solution (oC)
(∆T), Boiling point elevation (oC)
Which of the solutions exhibited the greater boiling point elevation? Explain why.
Exercise 15 Page 8
Exercise 15 Page 9
Illinois Central College
CHEMISTRY 130
Name:___________________________
SHOW YOUR WORK
PRELAB: Exp.15 Boiling Point Elevation
1. A solution contains 5.60 grams of urea (a nonelectrolyte), CON2H4 , in 50.0 grams of water.
Calculate the molality of the solution.
2. Calculate the boiling point of the solution in Problem #1. (kb for water is 0.512oC/m)
3. Calculate the freezing point of the solution in Problem #1. (kf for water is 1.86oC/m)
4. A solution containing 18.50 grams of an unknown nonelectrolyte dissolved in 100.0 grams of
water boils at 101.25oC. What is the molecular mass of the unknown substance?
Exercise 15 Page 10