Boiling Point Elevation
Safety Considerations: The solutions we are working with today are not hazardous, but we will
be boiling them, so there is a heat hazard. Be careful near the hotplate. Use your crucible tongs
to handle the hot glass. There is an old saying in laboratory science – “Hot glass looks like cold
glass.” So, be careful.
Objective: We want to measure the changes in boiling point of various aqueous solutions
relative to that of pure water. We specifically hope to see how the change depends on solution
concentration and the ionic concentrations.
Background:
Solutes added to a solvent will 1) lower the freezing point, 2) raise the boiling point, and 3)
increase the osmotic pressure of pure solute trying to dilute the solution. These three properties
are referred to as colligative properties. These properties depend only on the identity of the
solvent and the concentration of the solute. That is, solutions of NaCl and KNO3 that have the
same concentrations raise the boiling point the same amount. The chemical differences between
NaCl and KNO3 are not important. The only aspect of the solute that is important is the number
of particles it creates when it dissolves. A solute that produces 3 ions (per formula unit) when it
dissolves will increase the boiling point 3 times as much as a solute that only produces one
particle per formula unit.
The changes in boiling point are described by the boiling point equation:
∆𝐵𝑃 = 𝑘𝐵𝑃 × 𝑚 × 𝑖
where kBP is the boiling point constant for the solvent being used,
m is the molal concentration, and
i is the van’t Hoff factor, which is basically the number of particles each formula unit
breaks into on dissolving.
Most people find this a bit surprising that the extent of boiling point elevation (or freezing point
depression or increase in osmotic pressure) doesn’t depend on the identity of the solute but only
one the identity of the solvent. Well, in case you are surprised too, this lab is supposed to
demonstrate the truth of the statement.
Specifically, you and your classmates will determine the boiling points of solutions of NaCl,
KNO3, Na2SO4, and sucrose. We expect NaCl and KNO3 to dissociate into 2 ions, Na2SO4 to
dissociate into 3 ions, and the sucrose should remain undissociated when dissolved. Based on
the discussion above, you should try and predict the similarities and differences of how the
boiling points should change for the different solutes at different concentrations.
In carrying out this experiment we will simply measure the temperature of boiling solutions.
Unfortunately, as a solution boils, solvent evaporates but the solute doesn’t (at least not for
nonvolatile solutes, as we have today). This means the concentration increases with time, so the
boiling point increases. To determine the boiling point of the desired solution we will keep track
of the temperature of the boiling solution with time and extrapolate the changing temperature
back to when the boiling started. The following plot shows typical behavior of the temperature
of a boiling solution and where we assign the desired boiling point.
Today you will use a temperature probe that hooks into your computer. It is considerably more
precise than the thermometers in your lab drawers. A general description of the interface and the
program needed to run it is included on the following pages.
Important Lab Techniques: PasPort Interfaces and DataStudio Software
We have a collection of probes that can be connected to your computer so that you can make
measurements and have them directly exported to your computer. This gives you a great deal of
control over the presentation of your data. The probes we will use in CHM111L include a
temperature probe and a pH probe. They get connected to an interface which has a USB
connector that gets plugged into an appropriate port on your computer as shown in the figure
below.
Data collection is controlled with a program called DataStudio that can be found in the standard
load that came with your WFU laptop.
To start the program click on…
Start – All Programs - WFU Academic Tools – Scientific Tools – Data Studio(folder) –
DataStudio
Then click on “Create an experiment”, and close the “Digits 1” window.
Double click on any display you want to see from the DISPLAYS window. In this lab you want
to see the Digits (a digital display) and a Graph. After you double click on the type of display,
select the probe you want to display – temperature in this lab.
When you are ready to start measuring, click the “Start” button at the left of the top menu line.
When you are ready to stop, click “Stop”.
You can Export data to a text file by clicking on File-Export. Before you do that, however, you
will want to make sure it is going to export all the decimal places you want. Double click on the
Temperature in the Data window. There will be a Numeric tab in the window that pops up.
Select the desired number of decimal places, and click OK. Then you are ready to export a
selected set of data.
There are many other features of the data collection you can control, but these will serve to get
you going in this lab where we are using simple temperature measurements.
Procedure:
-Set up the apparatus as shown in the figure below. It involves a 125mL Erlenmeyer flask
containing 10-12g of boiling chips, and stoppered with a #4 one-hole stopper. The flask sits on a
hotplate. A temperature probe is draped into the flask through the stopper hole.
DO NOT PUSH THE STOPPER IN TIGHTLY. IT NEEDS TO BE IN LOOSELY, SO THAT
THE STEAM CAN ESCAPE.
- Put 25mL of distilled water in the flask, and turn on the heat. Monitor the temperature with
DIGITS and a GRAPH at least one minute beyond the time when boiling begins. Estimate the
boiling point by extrapolating the GRAPH temperature back to where the boiling started.
- Take the flask off the hotplate, and after it cools a bit, drain the water from it. Do not remove
the boiling chips.
- Take a 50mL sample of your solution (2 molal of one of the solutes). Dilute 12.5mL to 25mL
with water (this will be approximately 1 molal). Put this 1 molal solution into the Erlenmeyer
flask.
- Turn the heat on and monitor the temperature with DIGITS and a GRAPH until at least one
minute after the solution starts boiling. Estimate the boiling point by extrapolating the GRAPH
temperature back to where the boiling started.
- Take the flask off the hotplate, and after it cools a bit, drain the solution from it. Rinse the flask
with about 10mL of the 2 molal solution, and drain it.
- Add 25mL of the 2 molal solution to the flask, and measure it’s boiling point as you did for the
previous solutions.
-Repeat this procedure with another solute.
Data Treatment:
-Calculate and report the ΔBP for the two concentrations of your two solutes. Collect all the
class’s results.
- Produce a table of the BPs and ΔBPs for all the solutions tested.
- Plot ΔBP vs molal concentration for each of the four solutes. Add trendlines, forced to go
through the origin. [Freely delete data points that lie away from the other points for the same
condition.]
- Tabulate the slopes. Divide each of the slopes by the smallest slope. Tabulate the ratios.
Be sure your lab report includes…
- an abstract including an objective, a brief mention of the technique, and the final results. The
results should include any conclusion you can make about the dependence of ΔBP on
concentration and the number of ions made when the solutes dissolve.
- your table of the BPs and ΔBPs for all the solutions tested.
- your plot of ΔBP vs molal concentration for each of the four solutes, including trenslines
- your conclusion of the dependence of ΔBP on the solute concentration and on the identity of
the solute.
- a prediction of kBP for water, i.e. the boiling point elevation for 1 molal solutions in which the
van’t Hoff factor is 1.
- answers to the following two questions:
In the interest of time, we did not have you thoroughly drain and rinse one solution
before using the next. What effect would you think that might have on the measured boiling
point of a 1.00 molal solution if you measured it’s boiling point right after measuring the boiling
point of pure water, but you weren’t able to get all the water out of the flask before adding the
1.00 molal solution ?
If you make a solution from 100.0g of an unknown solute and 1000mL (1 kg) of water
and find that its boiling point is 1.024oC higher than that of pure water, what can you conclude
about its molecular weight ?