Determination of Molar Mass by Boiling Point Elevation of
Urea Solution
CHRISTIAN E. MADU, PhD AND BASSAM ATTILI, PhD
COLLIN COLLEGE
CHEMISTRY DEPARTMENT
Purpose of the Experiment
Determine the boiling point of pure Ethanol and its Kb using
Urea solution. Determine the molar mass of an unknown by
measuring the boiling point elevation of the unknown/ethanol
solution
A solution describes a system in which one or more substances (SOLUTE) are homogeneously
dissolved in another substance (SOLVENT). The proportion of the solute and the solvent in the
solution varies, with the solvent usually in greater quantity.
Physical properties are divided into two categories extensive properties, such as mass and
volume, depend on the size of the sample and intensive properties, such as density and
concentration, are characteristic properties of the substance that do not depend on the size of the
sample. There is a third category of properties which is a subset of the intensive properties of a
system. This third category, termed colligative properties, is applicable only to solutions.
When a solute is dissolved in a solvent, it lowers the freezing point of the solvent and it will
elevate the boiling point of the solvent. The degree to which the freezing point is lowered or
boiling point is elevated depends on the concentration of the solute particle. Properties that
depend on the concentration, but not the identity of the solute in solution are called colligative
properties.
Examples of colligative properties are:
vapor pressure lowering of a solution
boiling point elevation
freezing point depression
osmotic pressure
The following graph shows the phase diagram of a solvent and a solution in which the boiling
point elevation and freezing point depression are clearly shown.
Copyright © 2012 Christian Madu and Bassam Attili
1
Antifreeze; ethylene glycol aqueous solution in a car radiator is a typical example of the
application of the colligative property. In the advertisements it's called “summer protection
against boil-over.” Essentially what they have done is raised the boiling point of the water by
making it a solution.
In this experiment we will be measuring the boiling point of pure ethanol and the boiling point of
the Urea solution. From the difference in the boiling point, the boiling point elevation constant of
ethanol will be calculated. We can relate the boiling point elevation of a pure solvent ( Tb) to the
molality of the solution (m) as shown in the equation below:
Tb = T (solution) – T (solvent) = Kb m
Kb = boiling point elevation constant, which is a function of the solvent and not the solute.
Table 1 shows Kf‘s and Kb’s for a list of solvents. You may use the table to do the pre-lab
calculations.
m = molality, it is calculated as follows
Copyright © 2012 Christian Madu and Bassam Attili
2
Expressing the number of mole in terms of its mass and its molar mass, we can rewrite the
molality equation in the following manner:
mass of
solute,(g)
1 mol of solute
molar mass of solute, (g)
m = molality, (mol/Kg) =
Mass of solvent, (Kg)
We can then use the boiling point elevation measurement to determine the molar mass of an
unknown solute.
Table 1:Boiling Point Elevation and Freezing Point
Depression Constants of Several Solvents at 1 atm.
Solvent
Boiling Point C
Kb
Melting Point C
Kf
117.90
80.10
46.20
76.05
61.70
34.50
100.00
3.07
2.53
2.34
5.03
3.63
2.02
0.51
16.60
5.50
-111.50
-23.00
-63.50
-116.20
0.00
3.90
4.90
3.83
30.00
4.70
1.79
1.86
Acetic Acid
Benzene
Carbon Disulfide
Carbon tetrachloride
Chloroform
Diethyl Ether
Water
Procedure:
2
3
0
Caution: Ethanol is flammable, keep away
from open flames. Use stirring hot plates to
heat the water bath.
1. First prepare a 90 °C water bath by heating 400 mL of water in a 600 ml beaker.
2. Determine the mass of Urea (60.06 g/mol) necessary to make 50 mL of 1.0 molal
solution in Ethanol (d = 0.804 g/ml). Measure out three Urea samples of the calculated
mass. Show your calculation to the lab instructor before continuing.
Copyright © 2012 Christian Madu and Bassam Attili
3
3. Measure 50.00 mL of ethanol and place it into a clean and dry an 8 x 1-in test tube. Place
a small capillary tube (about 3-4 inches long) in the ethanol, open end down. The test
tube is then fitted with a two holed rubber stopper with thermometer (or temperature
sensor) inserted so that the tip of the thermometer is about an inch below the surface of
ethanol, and an 8 inch length of glass tubing. The test tube is then clamped and immersed
in the hot water bath and heat gently.
4. Use this set up to determine the boiling point of pure ethanol.
5. Use the same ethanol and determine the elevated boiling point when one, two and three
molals of urea is added (measured in step 2).
6. Construct and record your data in a table form.
7. Construct a graph of T; boiling point (y-axis) vs. m; molality (x-axis). Show the equation
of the line and calculate the slope. Make sure to properly label the axes and title the
graph.
8. Get an unknown from your lab instructor. To a new batch of 50.0 mL ethanol, add a mass
of the unknown equal to the mass of one molal of Urea (calculated in step 2). Use the
above procedure to measure the boiling point elevation of the unknown/ethanol solution.
Calculate the molar mass of the unknown.
9. Given the density of ethanol (0.789 g/cm3), the mass of ethanol used may be found and
hence the mass of unknown in 1 kg of solvent. From these data and the molality, the
molecular weight of the unknown may be determined.
Pre-lab questions:
Should be answered and turned to lab instructor before starting the lab.
1. Define the term “colligative properties” and give examples.
2. What is molality?
3. Calculate the molality of a solution that contains 1.875 g of potassium chloride (KCl) in
175 g of water.
4. How much will the boiling point of water change when 2.00 g of urea (60.1 g/mol) is
dissolved in 150 g of water?
5. A solution of 100 grams of brucine in 1 kg chloroform freezes at –64.69 °C. What
is the molecular weight of brucine?
Copyright © 2012 Christian Madu and Bassam Attili
4
Critical Data and Discussion to Include in the Lab Report:
•
•
•
•
Data table that includes: Molal concentration, boiling point and change in boiling point
Detailed calculations from the data and properly labeled graph.
Discussion of the results and possible sources of error
Work cited
Copyright © 2012 Christian Madu and Bassam Attili
5