determination of molar mass by boling point elevation of urea solution

Determine the boiling point of pure ethanol and its Kb by using
urea solution. Determine the molar mass of the unknown by
measuring the boiling point elevation of the solution of the
unknown in ethanol
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 depends 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 antifreeze 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 liquid by making it a
Copyright © 2010 Christian Madu
In this experiment we will be investigating the boiling point elevation constant of ethanol.
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
Expressing the number of mole in terms of its mass and its molar mass, we can rewrite
the equation in the following manner:
mass of
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.
Caution: Ethanol is flammable, keep away from open flames. Use stirring hot
plates to heat the water bath.
1. Prepare first a 90 °C water bath by heating 400 mL of water in a 600 ml beaker.
2. Determine the mass of urea necessary to make 30 mL of 1.0 molal solution and
measure out three samples of the calculated mass. Show your calculation to the
lab instructor before continuing.
3. Clean and dry an 8 x 1-in test tube. Then using a volumetric pipet measure 30.00
mL of ethanol into the clean test tube. Place a small capillary tube (about 3-4 in
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 water
bath then gently heated.
Copyright © 2010 Christian Madu
4. Use this set up to determine the boiling point of pure ethanol and the elevated
boiling point when the different portions of urea are added. Construct and record
your data in a table form.
5. Construct a graph of T (y-axis) vs. m (x-axis). Show the equation of the line and
calculate the slope. Make sure you properly label the axes and title the graph.
6. Get the unknown from your laboratory instructor, and with a new batch of 30.0
mL ethanol, use the above procedure in step 1-4 to measure the boiling point
elevation of the ethanol solution and from your data calculate the molar mass of
the unknown.
7. Given the density of ethanol (0.785 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 may be determined.
Thought questions before performing 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?
Critical Data and Discussion to Include in the Lab Report:
• Data table that includes: Molal concentration, change in boiling point
• Detailed calculations from the data and properly labeled graph.
• Discussion of the results and possible sources of error
• Answers to all questions posed above
• Work cited
Copyright © 2010 Christian Madu
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