Name __________________________________________
Date ________________ Per ____
Group Members: _________________________________________________________________________________
Solutions – Colligative Properties – Boiling Point Elevation
Introduction: Compared to a pure solvent, the boiling point of a solution is higher. Why? Remember that the
solute lowers the vapor pressure by replacing the attraction between solvent molecules with its own IMF’s,
which blocks evaporation and condensation. If the vapor pressure is lower than normal, that also means we
need more energy to boil the solvent. Water usually boils at 100ºC here at sea level. With solute added, this
should increase, but by how much? The equation that we use to express this is:
ΔTb = Kb x m
The change in boiling point is related to what the solvent is (Kb), and the concentration of solute (m; molality).
While prepping for the lab, Mr. Capps tripped, breaking the jars and spilling all the compounds he was carrying.
Frantic, he scooped them all up into unmarked beakers, and then swept everything else into a corner. To hide
the evidence, he dissolved them all in water, so they just looked like they’re being washed. Now, he has no
idea which sample is which, and he’s not quite dumb enough to try tasting them to find out. Since you are his
willing (?) assistants, you can help him figure out which is which.
Objective: Measure the boiling points of different solutions, and use those measurements to determine what
the solutes could be. Relate the solute identity to the change in boiling point and calculate error.
Materials:
hot plate
250 mL beakers (4)
boil chips
stirring rod
100 mL graduated cylinder Beaker Tongs
Thermometer (alcohol or digital)
Unknown A, B, C
Distilled water
Goggles (and any other safety equipment)
Safety: First of all, hot plates are HOT. Take care to keep them at the center of the lab bench. Do not lean on
or near the hot plates, and do not pick them up or move them from the lab bench. Anything heated on the hot
plate will also be HOT, such as glassware. Use of beaker tongs is suggested if you need to move hot
glassware for any reason. Do not hover over boiling solutions, as vapor droplets could scald your skin or
inhaled vapors could be hazardous. Do not rinse hot glassware immediately, as it could crack or shatter. Allow
time for beakers to cool before cleanup.
Procedure:
1. Before doing any actual lab steps, complete the prelab questions on the final page.
2. Pour 100.0 mL of H2O into a beaker. Add two boil chips to the beaker (to break surface tension ->
this speeds up when you notice boiling happening).
3. Heat up the water to boiling, using your thermometer to measure the boiling point. *This is crucial,
as the thermometers may not be correctly calibrated. This way, if you get “weird” numbers for your
unknowns, you have a base boiling point to use for correction.*
B.P. of H2O _________________________
4. You will need to make 100 mL solutions of each unknown. Remember what we talked about in
class with the molarity worksheets, as to how to be as accurate as possible. Take 10 mL of each
unknown from the front table for these solutions. Label each beaker, so you don’t mix them up!
Volume of A: _____________
B: ___________________
C: __________________
5. Add two boil chips to each beaker, then heat up each sample to boiling. Measure the boiling point
of the solution. While waiting for the solutions to boil, this is a good time to work on lab questions or
other class work. However, at least one person per group should be designated to watch the
solutions.
B.P. of A ________________
B: ___________________
C: __________________
6. Using beaker tongs, carefully remove the hot beakers from the hot plate, setting them aside to cool
before cleaning them. When all samples are measured, turn off the hot plate and let it cool down on
its own.
7. When every group is done, the identities of the three unknowns will be given, and values for error
calculations. Determine which unknown was which through the change in boiling point:
ΔTb = BP Unknown (Step 5) – BP H2O (Step 3)
∆Tb A: ________________
∆Tb B: _____________
∆Tb C: _______________
Identity of A: ________________
B: _________________
C: ___________________
8. Calculate the error for each of your samples.
Experimental  Accepted
x 100% = Percent Error
Accepted
Error in A: ________________
B: _________________
C: ___________________
9. No group will receive full credit unless their lab station is cleaned up and ready to be used
by the next class!
Prelab Questions:
1. In the boiling point elevation equation, what is m?
2. How do the boil chips help in this experiment?
3. Why are we boiling a sample of water?
4. Calculate the concentration of particles in solution if 0.0300 moles of an ionic solid, such as CaCl 2,
dissolve in 15.0 mL of water.
5. What dangers are there in this lab? (Hint: You’re working with unknowns!)
Postlab Questions:
1.
Were there any differences in the rate of boiling for your three solutions? If so, why would there be
a difference? If not, why should they all heat at the same rate?
2.
Why was there a difference in the boiling points of the three solutions?
3.
What would be the boiling point of a solution of FeBr3 in H2O (same concentration as in lab)?