Honors Chemistry: Colligative Properties of Solutions: A Study of Boiling Point Elevation
(AN INFORMAL LAB)
OBJECTIVES: In this experiment, you will:
a) Determine the boiling point elevation constant for water and the molecular weight of an
unknown. You will do this by observing the boiling points of solutions of known solute and of
solutions with an unknown solute.
b) Calculate to calculate the value of the boiling point constant for water through experimentation.
c) Use colligative properties to determine the molecular weight of a substance.
SAFETY: goggles, closed toed shoes, lab apron, clean all lab equipment before and after
MATERIALS & CHEMICALS:
400mL Beaker
sodium chloride Deionized Water
Bunsen burner Unknown Substance
Table of selected solvents:
Solvent
Acetic acid
Ethanol
Water
FP (o C)
16.6
-114.1
0.0
Kf (o C/m)
3.90
1.99
1.86
BP (o C)
118.1
78.4
100.0
Kb(o C/m)
2.93
1.22
0.51
Pre -lab questions: (to complete before coming to class)
1. Colligative properties are those properties of a solution that depend on the number of
molecules or ions dissolved in a solution, and not on the identity of the species in solution.
Define and give an example of each of these properties: boiling point elevation, freezing point
depression, and osmotic pressure.
2. This week, you will measure the boiling point elevation of salt water solutions. With that
information, you should be able to calculate the van’t Hoff Factor from plotting the data and
drawing a trend line to connect the points. Please write out the equation that relates the boiling
point elevation to the van’t Hoff Factor, and identify what each variable in the equation
identifies.
3. Define the units of molality and identify in this lab which component was the solute and the
solvent?
4. Show Calculation: For the 2.0 molal copper (II) chloride salt in 500-g of acetic acid, calculate your
average boiling point, the van’t Hoff factor, and your calculated boiling point constant (show
calculations and box your final answer with correct units and sig figs)?
5. Pre-lab expectations:
 Highlight all action terms and data to be collected in the procedure
 Do not forget to do all calculations and graph expectations in the procedure (*see
highlighted sections).
PROCEDURES:
A: Determining the Boiling Point Constant of Water
1. Calculate the mass of sodium chloride necessary to make about 200mL of a 1.0-m solution. Show your
calculation to your teacher before coming to class
2. Measure out three samples of salt of the calculated mass.
3. Bring 200mL of deionized water to boil in a 400mL beaker. Record the temperature of the water.
4. Carefully add the first sample of salt to the boiling water and allow to boil. Make sure you do not have
salt sticking to the sides of the beaker and that it is all dissolved. Record the temperature and the
molality
5. Carefully add the second sample to the boiling solution and allow to boil. Record the temperature and
molality
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6. Carefully add the third sample to the boiling solution and allow to boil. Record the temperature and
molality
7. Construct a graph of T (y-axis) vs. m (x-axis) before coming to class so that you can use your lab time
to plot the graph Do not forget to give the graph a title, label both x and y-axis with proper variables and
units. Calculate the slope, intercept, and equation of the graph. Show your calculations on the graph
(with formula, units and sig figs).
B: Determination of the Molecular Mass of an Unknown
1. Record which unknown you are using.
2. Measure out one sample of the unknown roughly equal to the mass of one of the samples you used in
part A.
3. Bring 200mL of deionized water to boil in a 400mL beaker. Record the temperature of the boiling
water.
4. Carefully add the unknown sample to the boiling water and allow boiling. Record the temperature on
your student designed data table.
5. Calculate the molecular weight of the unknown salt – show all formulas, calculations and units. Your
instructor will tell you how many ions per formula are produced for your particular unknown.
REPORT FORM FOR COLLIGATIVE PROPERTIES LAB
PART A: Determining the Boiling Point Constant of Water
Volume of Water ____________________
Mass of salt sample 1____________________
T ___________________
m____________________
(show molality calculation)
Mass of salt sample 2_____________________
T____________________
m____________________
(show molality calculation)
Mass of salt sample 3______________________
T____________________
m____________________
(show molality calculation)
** Do not forget to: Attach graph of T vs. m. with all expected calculations**
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PART B: Determination of the Molecular Mass of an Unknown
Unknown ____________
#Ions/Formula______________
Mass of Unknown______________________
Volume of water______________________
Boiling T___________
T after addition of unknown sample___________________
Molecular weight of Unknown_____________________
(show molecular mass calculations)
Post lab questions (ANSWER ON A SEPARATE SHEET OF PAPER)
1. Give at least 3 ‘real world’ uses of colligative properties.
2. Calculate the heat needed to boil away your salt water sample (PART A – after all 3 salt samples)
3. How does changing percent concentration of antifreeze affects the boiling and freezing points of
the solutions? Explain what percent solution would be most effective in preventing freezing and
boiling in your car’s radiator based on the average lowest and highest temperate in Marietta, GA
this school year. Describe how you came to this conclusion. (YES – you will have to do some
research to answer this question and attach a copy of the weather information collected to
respond to this question)
4. From the graph below, determine the KB for the unknown solution. (van’t Hoff Factor = 5) (show
calculations)
CONCLUSION: Paragraph format on a separate sheet of paper. Please refer to your data collected,
introduction terms/definitions and the objectives of this lab experiment when explaining.
ESOE: Paragraph format on a separate sheet of paper. Explain at least 2 sources of error based on data
collected and compare to the actual. Explain using % error where possible.
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