Charles' Law
According to Charles' Law the volume of a gas is directly proportional to the Kelvin temperature assuming the pressure
is constant or
V = kT where V = volume, T = the Kelvin temperature and k = a constant number
If we have two different temperatures, T1 and T2 Charles Law can be restated as
V1
=
T1
V2
T2
Boyle's Law states that the pressure and the volume of a gas are inversely proportional to each other assuming the
Kelvin temperature remains constant or
PV= k where P = pressure, V = volume and k = a constant number
If we have two different pressures, P1 and P2 Boyle's Law can be restated as
V1
=
V2
P2
P1
Charles Law and Boyle's Law can be combined in the following relationship:
PV
=k
T
If we have two sets of conditions the combined law can be restated as
P1V1
=
P2V2
T1
T2
In this experiment you will check the validity of Charles Law by measuring the volume of a sample of air at two different
temperatures. Using Charles Law you will predict the volume the gas should be at the lower temperature based on its
volume at the higher temperature. Then you will compare the predicted volume at the lower temperature with the actual
measured volume .
Since Charles law predicts the ratio of volume to temperature is constant, you will also calculate the volume to
temperature ratios for the two temperatures and see if they agree.
For the purpose of this experiment, the volume of air at the higher temperature is assumed to be dry, although it will
actually contain a small amount of water vapor due to atmospheric humidity. The volume of air at the lower
temperature is measured over water. Because this volume of air is saturated with water vapor that contributes to the
total pressure in the flask, the volume must be corrected to that of dry air at atmospheric pressure. This correction can
be done by first subtracting the vapor pressure factor of water at that temperature from the total pressure and then
calculating the volume at atmospheric pressure that the dry air would occupy using Boyle's Law
Materials
Erlenmeyer Flask, 250 cm3
Thermometer
Beaker, 600 cm3
Graduated cylinder 100 cm3
Rubber stopper fitted with a pinch clamp
Large dish pan
Ring stand and clamp
Hot plate
Procedure
Put a 600 cm3 about 2/3 full of water on the hot plate to boil. Meanwhile dry a 250 cm3 Erlenmeyer flask gently using a
paper towel. If necessary put the flask briefly on the hot plate. Then allow the flask to cool. While it is cooling fit it with
the rubber stopper and pinch clamp assembly. Insert the stopper into the flask and then gently immerse the flask into
the boiling water as far as possible and clamp it in place with the clamp. Make sure that the rubber tubing coming out of
the stopper is open See diagram below:
Continue to heat the water until it is boiling. Keep the flask in the boiling water for at least 8 minutes to allow the air in
the flask to attain the same temperature as the boiling water. Add water as necessary to maintain the level of water in
the beaker. Read and record the temperature of the boiling water.
While the flask is still in the boiling water pinch off the rubber tubing in the stopper using the pinch clamp and remove
the flask from the boiling water and completely immerse it in a dish pan of cold water. With the flask completely
submerged, open the pinch clamp under water, letting cold water flow into the flask. Keep the flask totally submerged
for 4-6 minutes to allow the flask and its contents to attain the temperature of the water in the dish pan. Read and
record the temperature of the water.
In order to equalize the pressure of the air in the flask with that of the atmosphere, bring the water level in the flask to
the same level as the water in the pan by raising or lowering the flask. See the diagram below. With the water levels
equal, pinch the clamp to close the flask. Remove the flask from the water and set it on the lab bench.
Using a graduated cylinder measure and record the volume of water that was drawn into the flask. Then fill the flask
completely with water and measure the total volume of the flask by measuring the volume of water that it holds.
Dry the flask and repeat the experiment again as time permits.
Data
Data
Trial 1
Trial 2
1. Atmospheric Pressure. (record
from Horizons computer or the
barometer in room 216)
2. Temperature of cold water
3. Vapor Pressure of water for
above Temperature
4. Temperature of boiling water
5. Volume of the flask
6. Volume of water absorbed by
the flask
Calculations
1. Calculate the pressure of the air at the lower temperature by subtracting the vapor pressure of water at the lower
temperature from the atmospheric pressure. ( Line 1 - line 3)
2. Calculate the volume of wet air at the lower temperature by subtracting the water absorbed from the total volume of
the flask. ( Line 5 - line 6)
3. Using Boyle's Law correct the volume of wet air to what it should be if it were dry
Volume of dry air at low temperature = Volume of wet air x Pressure of dry air
Atmospheric Pressure
4. Predict the expected volume at the lower temperature from the volume at the higher temperature using Charles'
Law:
Predicted Volume at low temperature =
Volume at high temp X
Lower temperature (in Kelvin)
Higher Temperature (in Kelvin)
5. Calculate the percent error between calculation 3 and calculation 4 above, using calculation 4 as the true value.
6. Compare the volume / temperature ratios for the two temperatures.
7. On graph paper plot the temperature and volumes for high temperature and the low temperature, putting the
temperature in Celsius on the x axis. Use values from -350°C to + 400 °C on your x axis. Draw a straight line between
the two data points and extend it until it crosses the x axis. At what temperature does it cross? What is the significance
of this point?
Sample Calculations
Charles Law Experiment Notes on Processing the data
The following is a Sample Data Table. The data was made up for purposes of illustration.
Sample Data
*Atmospheric pressure
Volume of the Flask
Temperature of the
boiling water
Temperature of the cold
water
Volume of water
absorbed by the flask
Volume of air at the low
temperature
*Vapor Pressure of
water at 21 degrees
765.0 torr
280 cm3
100.0 oC
21.0 oC
60.0 cm3
=280-60
=220 cm3
18.7 torr
Pressure Units
*Pressure in this example was measured in mm of mercury or torr. If your measured the pressure
in inches of mercury multiply it by 25.4 mm per inch to get the pressure in torr. If you used millibars
divide by 10.5 to convert your pressure to kilopascals (kPs). If you want to convert torr to
kilopascals divide by 7.5. You may use either torr or kPa for pressure units in these calculations
*The vapor pressure of water is available from Chemtables which I have loaded on the computer
under the periodic table in room 217 or from your book.
Calculations Step by Step
1. The air at the lower temperature is wet. Convert it to what the volume would be if it were dry. The
process is as follows:
• First subtract the vapor pressure of water from the atmospheric pressure
765.0 torr- 18.7 torr = 746.3 torr.
• Multiply the volume at the lower temperature by a ratio of the pressure at the low
temperature over the pressure at the high temperature
220 cm3 x (746.3 torr / 765 torr) = 214.7 cm3
2. Calculate what the predicted volume of the air would be at the low temperature based on
Charles Law. The process is as follows:
• Convert your temperatures to Kelvin
100oC + 273 = 373 K
21oC + 273 = 294 K
• Multiply the volume of the flask by a ratio of the low temperature divided by the high
temperature
280 cm3 X (294K / 373K ) = 220.7 cm3
3. Compare the two values:
220.7 cm3– 214.7 cm3 = 6.0 cm3
Percent error = (6.0 cm3/220.7 cm3) x 100 = 2.7%
4. Plot a graph with temperatures on the x axis and volume on the y axis. Extrapolate the line to
find the x intercept. This value should correspond to absolute zero. In the sample data above you
would use
T1 = 100 oC, V1 = 280 cm3
T2 = 21 oC V2 = 214.7 cm3