The Gas Laws
In the seventeenth century, Robert Boyle investigated the relationship between the volume of
a confined gas and the pressure it exerted upon its container. He found that these variables were
related as an inversely proportional function. He expressed this function using the formula P x V =
k (for a constant number of moles at constant temperature). This relationship can be explained in
terms of the kinetic molecular theory. As the volume of the gas is decreased, more collisions occur
per unit area of the container walls, thus increasing pressure. Similarly, when volume is increased,
fewer collisions occur per unit area, and a drop in pressure is observed.
More than 100 years later (late 1780s), Jacques Charles observed a relationship between the
volume of a gas and its temperature. He found that as a sample of gas was heated its volume
increased. In terms of the kinetic molecular theory, as a gas is heated its molecules move at greater
velocity and are capable of occupying a larger volume while maintaining the same pressure.
Charles’s work led to the formation of the absolute temperature scale, a measuring system based on
a more direct relationship between molecular motion and temperature.
In this experiment, you will attempt to duplicate the experiments of Boyle and Charles. A
confined gas will be subjected to different pressures and temperatures. The resulting change in
volume will be recorded. You will then plot these results and extrapolate the volume-temperature
graph to find the volume that corresponds to zero on the absolute temperature scale (Kelvin).
O
B J E C T I V E S
1. Observe the effect of increasing pressure on the volume of a container.
2. Observe the effect of increasing temperature on the volume of a confined gas.
3. Construct a volume-temperature graph from the collected data and determine from the graph
the volume of a gas at absolute zero.
Materials
Gas piston–cylinder assembly
with block supports
thermometer
ring stand and ring
Utility Clamp
Barometer
Bunsen burner
centigram balance
600 ml beaker
Erlenmeyer flask
wire gauze
5 weights
metric ruler
kilogram or pound scale
One hole stopper with tube
Procedure
-
Part 1
Boyle’s Law
1. Measure the barometric pressure of the room.
Convert this reading to kPa and record this
value in Data Table #1.
2. Obtain a gas piston-cylinder with block
supports. Separate the piston-cylinder
part, shown in Figure A, from the rest of
the assembly. Remove the cylinder cap,
then separate the piston from the cylinder
and measure the internal diameter of the
cylinder in cm. Record this measurement
in data table #1
3. Measure the mass of the piston (refer to
figure A for which part is the piston) plus
the upper support block (see Figure B)
using the centigram scale. Convert this
value to kg. Record this value in data table
#1
Figure A
4. Reassemble the piston with the cylinder. Fill
the cylinder to 30 ml with air. Replace the
cap and assemble the piston-cylinder with
the upper and lower supports.
5. Label five weights (probably old books) A
through E. Deter mine the mass of each
using the kilogram or pound scale (2.2 lb.
= 1 kg or 454 g = 1 lb.). DO NOT USE
THE CENTIGRAM SCALE. Record
these masses in data table #2.
6. While making sure the weights do not fall
over place weight A on the upper support
block of the gas piston. Record the
volume of the confined gas in data table
#3.
7. Add additional weight one at a time, and
record the subsequent gas volume.
Continue until all five weights have been
stacked.
Figure B
Procedure - Part 2
Charles’ Law
1. Set up the apparatus as shown in the
figure to the right. Obtain a 600-mL
beaker and add approximately 250
mL of tap water. Obtain a 250 mL
Erlenmeyer flask. Place a one–hole
stopper fitter with a three cm long
piece of glass tubing (or dropper
pipette) in the flask. CAUTION:
Make sure the ends of the glass
tubing are smooth.
2. Place the flask in the beaker as shown.
Make sure there is some space
between the walls of the beaker and
the flask to allow steam to escape.
Heat the water to a vigorous boil.
Record the temperature of the boiling
water as T1 in data table #4.
Continue to heat the water at a
vigorous boil for three to five
minutes.
3. Half fill a pneumatic trough with tap water. Leave the clamp attached to the flask until it is
submerged in the trough. CAUTION: Flask is hot. Using appropriate protection such as hot
hands, place you finger firmly over the glass tube. It is essential that you use your finger to seal
the tube until it is submerged in the trough. Do not turn off the gas until your finger in on the
tube.
4. Invert and submerge the flask in trough. Remove your finger from the tubing. Water will rush
into the flask. While this is happening remove the clamp. When water is done rushing in,
raise the level of the flask so the level of the water in the flask is even with the level of the water
outside the flask. This will cause the air pressure to be equal inside and out (constant pressure).
5. With the levels equal, place your finger back over the tubing and remove the flask from the
trough, placing the flask upright on the bench. Remove the stopper and record the temperature
of the water inside flask (T2). Using a graduated cylinder, measure the volume of the water in
the flask (V1).
6. Refill the flask with water to the very top. Insert the rubber stopper allowing excess water to flow
out. Remove the stopper and measure this volume of water with a graduated cylinder (V2).
7. Repeat experiment, except this time, after the water has finished rushing in, cool the water in the
trough by adding ice to drop the temperature to as close to freezing as you can get. You do not
need to measure the volume of the entire flask again.
8. Before leaving the laboratory - please clean up all materials and wet wipe your lab bench. Wash
your hands thoroughly.
Name______________________
The Gas Laws
Pre-lab
1. State Boyle’s Law in your own words
2. Give the equation that describes the relationship between temperature and volume
3. What is meant by the term elastic collision
4. In part 2, at what temperature will the final reading be taken?
5. In step 5 of Part 2, why should the thermometer bulb be held at a point at which the bulb is even
with the midpoint of the column of air?
6. Prior to drawing the air into the cylinder in Part 1, what must be done?
7. What is meant by the term extrapolation?
8. How do you convert Celsius temperature into Kelvins?
9. What is the conversion factor for kilograms and pounds?
10. State the Kinetic Molecular Theory.
Data Table #1
Barometric Pressure in
units on barometer
Barometric Pressure in
kPa
Internal Diameter of
Cylinder (cm)
Mass of Piston-Block
Assembly (kg)
Data Table #2
Mass of
Weight A
Mass of
Weight B
Mass of
Weight C
Mass of
Weight D
Mass of
Weight E
in pounds
in kilograms
Data Table #3
Volume of cylinder with no weights (mL)
Volume of Cylinder with weight A(mL)
Volume of Cylinder with weights A+B (mL)
Volume of Cylinder with weights A+B+C (mL)
Volume of Cylinder with weights A+B+C+D (mL)
Volume of Cylinder with weights A+B+C+D+E (mL)
Data Table #4
Temperature (°C)
Temperature (°C)
Temperature of Boiling Water (T1)
Temperature of Room Temp. Water (T2)
Temperature of Ice Water (T3)
Volume (mL)
Volume of water in Cool Flask (V1)
Total Volume of water in Flask (V2)
Volume of water in Ice Cold Flask (V3)
Temperature (K)
Calculations – part 1
Pressure is Force per unit area. Force is mass times acceleration. There are several
calculations you must do to calculate the total pressure on the volume of gas in the piston.
1. Convert the diameter of the cylinder into meters.
2. Calculate the cross sectional area of the cylinder in meters using A = pr2.
3. The force exerted due to each weight is mass times acceleration. The acceleration due to gravity
is 9.81 m/s2 . The force (and later the pressure) of the piston and top support block must also
be accounted for. Multiply the mass of the piston and block (Data Table #1) by 9.81 m/s2 .
Then multiply the mass of each book by 9.81 m/s2 . Record the results in calculations table #1.
Calculations Table #1
Force of
Piston+Block
(kg•m/s2)
Force of
Weight A
(kg•m/s2)
Force of
Weight B
(kg•m/s2)
Force of
Weight C
(kg•m/s2)
Force of
Weight D
(kg•m/s2)
Force of
Weight E
(kg•m/s2)
4. Pressure = Force/area. To calculate the Pressure due to the piston and block and also for each
individual book, divide the force the piston and the block and also each book by the cross
sectional area from calculation #2. This will give you pressure in Pascals. Covert this number
into kPa and record your result for each column in calculation table #2.
Calculations Table #2
Pressure of
Piston+Block
(kPa)
Pressure of
Weight A
(kPa)
Pressure of
Weight B
(kPa)
Pressure of
Weight C
(kPa)
Pressure of
Weight D
(kPa)
Pressure of
Weight E
(kPa)
5. Transfer the volume reading from data table #3 to calculation table #3 below. For Boyle’s Law
calculations the TOTAL pressure for each volume must be calculated. The total pressure with
no weights on top of the piston is simply the pressure due to the piston block assembly plus the
atmospheric pressure. The total pressure with each subsequent weight added is simply the total
pressure due to the pressure of piston block assembly and the atmospheric pressure plus the
pressure for each book that is on the pile. Calculate the total pressure for each volume reading.
Record your calculation in calculations table #3.
6. Total pressure times volume should be a constant. In calculations table #3 multiply volume
(column 2) times total pressure (column 3) for each row in calculations table #3 and record the
result in the last column of calculations table #3.
Calculations Table #3
Weights
Volume of Confined
Air (mL)
Total Pressure (kPa)
No Weights
Weight A
Weights A+B
Weights A+B+C
Weights A+B+C+D
Weights
A+B+C+D+E
Calculations Table #4
Volume (mL)
Volume of Cool Air (V2 – V1)
Volume of Ice Cold Air (V2 – V3)
k = PV
Analysis and Conclusions
1. Enter your data for pressure vs. volume into a computer graphing program. Plot and connect the
data points with a BEST FIT curve. Attach the graph to the end of this report.
2. What type of relationship does this illustrate? Explain.
3. Enter your data for temperature vs. volume into a computer graphing program. Plot the points
with a BEST FIT LINEAR curve. Volume should be on the Y-axis. Extrapolate the curve
until volume reaches zero. Attach the graph to the end of this report.
4. What type of relationship does this illustrate? Explain.
5. Extrapolate the volume–temperature data graph to find the temperature at zero volume. What is
that temperature? Is your data consistent with Charles’s definition of absolute zero? Why or
why not?
6. Why must zero in Kelvin be obtained by extrapolation instead of direct observation?
Synthesis
1. If a helium filled balloon is released at the Earth’s surface, what is its eventual fate? Why?
2. Aerosol spray cans should never be thrown into fires or disposed of in incinerators. Explain.