LAB: Boiling Point Elevation
Name_____________
Block____Date_____
INTRODUCTION
The boiling point of a solution is different from the boiling point of a pure
solvent. Adding a non-volatile solute to a solvent elevates the boiling point. The
molality of the solution determines the temperature at which the solution will boil. One
molal (mol/kg) solution elevates the boiling point by a precise amount depending on the
solvent. If the solvent is water, the increase will be Kb = 0.52oC/m.
ΔTb = Kbm
As ionic solids dissolve, they dissociate, resulting in at least two moles of
particles in solution for each mole of crystalline solute. The molal concentration of the
ions in solution increases (doubling, tripling, and so on). The boiling point elevation
changes accordingly. (Ex. 1 mol NaCl has 2 mol ions, [1 mol of Na+ and 1 mol of Cl-])
In this investigation, the boiling points of water, a sugar-water solution, and a saltwater solution will be determined.
PROCEDURE
1. Turn on the hot plate.
2. Measure out and pour 125 mL of water and pour it into a 250 mL beaker.
3. Record the initial temperature of the water and record it for the 0 min. reading in
the data table.
4. Begin heating.
5. Record the time and temp. readings at 30 second intervals. Record temp. to the
nearest 0.1oC. Continue until boiling (the temp. remains constant for 2 or 3
readings).
6. Empty and set aside the beaker to cool. Fill a different 250 mL beaker with 125
mL of water and add approximately 20 g of NaCl. Record the exact mass added.
Stir until the NaCl dissolves.
7. Repeat steps 3-5 for the NaCl solution.
8. Measure and record the mass of 6 sugar cubes. Using a different 250 mL beaker
add the sugar cubes to 125 mL of water. Stir until the sugar dissolves.
9. Repeat steps 3-5 for the sugar solution.
OBSERVATIONS
Mass of NaCl added _______g
Mass of sugar (C12H22O11)added __________g
Data Tables
CALCULATIONS
1. Calculate the molality of the NaCl solution.
2. Calculate the molality of the sugar (C12H22O11) solution.
3. Predict the new boiling pt. for the NaCl solution. (b.p. of water is 100oC)
(For the m value here, use molality of total ions. 1 mol NaCl = 2 mol ions)
4. Predict the new boiling pt. for the sugar solution.
5. Construct a graph using the data collected (Use an entire page). Plot temp. vs.
time. Plot the data for all three samples. Use different colors or symbols to
differentiate the three samples. Indicate the b.p.’s
6. Calculate the % error for the b.p. of the NaCl solution and the sugar solution
using the #3, #4 answers (the theoretical values) and your measured values (from
graph).
POST LAB QUESTIONS
1. Based on your results, out of ionic solids or non-dissociating solids, which has a
greater effect on the boiling point of water?
2. With regard to vapor pressure, explain why the boiling points of the solutions are
higher than the boiling point of water.