This lab is designed to demonstrate one of the fundamental
laws of gas behavior, Boyle’s Law. This law is about
the relationship between the pressure and volume of an
ideal gas when the number of moles and the temperature are
held constant. Furthermore, the law holds true regardless
of the identity of the gas. Credit for the first
publication about this law regarding gas behavior belongs
to Robert Boyle who wrote up his results in 1662. You will
use the Vernier Gas Pressure Sensor and a gas syringe to
vary volume and measure pressure to generate data to graph
in order to determine the nature of the proportion between
the pressure and volume of a gas.
By collecting data and graphing it you will determine
whether Boyle’s Law is a direct proportion or an
inverse proportion. A direct proportion is one in which two
variables are related in such a way that the value of the
dependent variable (usually plotted on the y-axis) equals
the independent variable (on the x-axis) multiplied by a
constant. The graph of such a proportion has the form of a
simple straight line and the equation for the line (in y =
mx + b form) shows how one variable is multiplied by a
constant (the slope of the line) to calculate the other. A
sample graph of a direct proportion is shown at right.
An inverse proportion is one in which two variables are
related in such a way that the value of the dependent
variable (y) equals a constant times the inverse of the
independent variable (1/x). The graph of an inverse
proportion has the form of a simple inverse curve and the
equation of the curved line has the form y = m(1/x). The
problem with this is that several functions have graphs
that look a lot alike. When using real data from a lab it
can be hard to be sure that the data are following the
equation y = m(1/x) or y = m(1/x2). These two
functions are hard to tell apart—try graphing them
using a graphing calculator. In order to confirm that your
data do follow a simple inverse proportion, just plot the
inverse of your independent variable values versus the
dependent variable. In other words, plug in the value of
1/x instead of x and graph it again. If it turns out to be
a simple straight line, then you can confirm a simple
This lab is designed to allow students to discover the
nature of the proportion between the pressure and volume of
a gas by collecting data and analyzing it using computer
software. Students will determine whether Boyle’s Law
is a direct proportion or an inverse proportion. They will
also learn to do simple proportional calculations.
20 mL plastic syringe
Vernier Gas Pressure Sensor and Interface
Computer with MS Excel
or other spreadsheet software
This lab is really very nearly risk-free
Boyle’s Law is a useful proportion that can be put to
work to answer questions about changes in volume and
Some Common Units and Conversions
1 atmosphere (1 atm) is the average air pressure at sea
level on Earth.
1 atm = 101.3 kPa = 1.013 × 105 Pa = 14.7
A typical car tire will have a pressure of about 35 psi
1 cubic meter (m3) is the SI unit of volume
1 m3 = 1,000 L 1 L = 1,000 mL
If a gas’s pressure is reduced by half at constant
temperature then its volume doubles. Here is how to use the Boyle’s Law proportion to calculate such changes.
First, let’s define the initial pressure and volume
as P1 and V1 and the pressure and
volume after a change as P2 and V2.
According to Boyle’s Law:
P1V1 = k1 and P2V2 = k2
Let’s assume those constants are the same (and they
will be as long as we do not add or remove any gas or
change the temperature). In that case:
P1V1 = P2V2
Now, here is a question we can answer using this
proportional equation: What is the final volume of a gas
when its pressure is reduced by half and its initial
pressure is 1.0 atm and its volume is 5.0 L?
Solving for V2 gives the answer 10 L. This is
exactly what we expected based on the idea that this is an
inverse proportion: when one variable is cut in half, the
other doubles. Use this example to help you to answer the
questions in the exercises below.
Do the following exercises neatly on a separate piece of
paper. Assume that temperature is constant for all of the
changes described in these problems. Also, assume that
pressure at the Earth’s surface is equal to 1 atm or
All problems can be solved using the correct form of
Boyle’s Law. This can be written as either PV = k or,
more usefully, as P1V1 =
If a gas in a volume of 25 mL with a pressure of 1
atmosphere (atm) is compressed to 5 mL what is its
If a gas with a pressure of 2 atm is confined in a
volume of 10 L what will its pressure be if the volume is
made to be 20 L?
A balloon with an internal pressure of 1.3 atm and a
volume of 2.5 L is placed into a vacuum chamber. What is
the balloon’s volume if the internal pressure is
reduced to 0.17 atm?
What is the new volume of a gas if the pressure of
the gas is reduced from 220 kPa to 100 kPa and the
initial volume was 1.5 m3?
At the bottom of the Challenger Deep in the Pacific
Ocean the pressure due to all that water overhead is 1091
atm. A bubble of gas with a volume of 1 mL is released by
an advanced submarine research vessel. What is the volume
of the gas bubble when it pops at the surface of the
ocean where the pressure is 1 atm?
A gas is confined in a bottle with a pressure of 5.2
atm. The volume of the bottle is 40 L. What volume would the gas have if it were stored at a pressure of 1 atm instead?
What happens to the pressure of a gas when its volume
is changed from 14 mL to 27 mL?
Why does the volume of helium in a weather balloon
increase as it rises from the ground to the
Remember to record your observations in your lab notebook
before you leave class.
Connect the LabQuest Mini to the computer and connect
the Gas Pressure Sensor to it. Start the Logger software.
Pull out the plunger of the syringe until the volume
reads 5 mL. Read the volume at the bottom-most black ring
on the rubber plunger where it touches the inside of the
Attach the syringe to the sensor by gently screwing it
Set up data collection as follows:
On the “Experiment” menu, select
Change Mode from “Time Based” to
“Events with Entry”.
Make the “Column Name” Volume
and the units mL. Then click “Done”.
(Pressure will be recorded using the SI unit of
pressure, the kilopascal (kPa): 1 atm = 101.3 kPa.
Right-click on the graph and select “Graph
Options…” at the top of the menu that pops up.
On the “Axes Options” tab change the box
marked “Right” to 20. This sets the maximum
value for the volume measurements which will not exceed 20
If you have a lab partner then have one partner manage
the syringe and have the other enter the data. Click the
big green “Collect” button to begin.
Move the piston of the syringe so that the volume
is exactly 2.5 mL and hold it in place.
Wait for the pressure reading to stabilize then
click the “Keep” button.
Enter the volume and click OK.
Repeat these steps at volumes of 5.0 mL, 10.0 mL, 12.5 mL,
15.0 mL, 17.5 mL, and 20.0 mL.
Click the big red “Stop” button.
Click the big “Save” button and write a
descriptive name for the data file. Store it on your
personal network drive. Alternatively, save it to the
desktop and then email it to yourself or upload it to your
personal document service.
Calculations and Graphing
Now that you have generated some data and taken a look at a
graph of it on your screen you will put that data in Excel
to produce a well-formatted graph and perform an analysis
to determine the exact nature of the proportion between the
pressure and volume of a gas.
Highlight the data table on the LoggerLite or LoggerPro
software screen. Copy by using
Paste the data into a spreadsheet program, leaving an
empty row at the top to add labels. Label the column on the
left Volume (mL) and label the column on the right Pressure
Insert a Scatter-plot graph of the data and format it
according to your teacher’s directions. The online
version of this lab has a link to a sample set of
Right-click the column heading for the column
containing your Pressure data (probably where it says
‘B’) and select “Insert” from the
menu that pops up. Label this column “Inverse Volume
(1/mL)”. In the cells of this column use a formula to
calculate the inverse of the Volume data. One possible
formula is “=A2^-1”. Copy the formula to all
the cells in the column next to Volume data.
You are about to create a graph to determine the exact
nature of the proportion. The first graph you made should
give you an idea of whether the data are likely to be
directly or inversely proportional. By graphing P vs. 1/V
you will see whether a straight line results. If it does
then it will confirm the proportion as a simple inverse
proportion that has the form P = k/V or PV = k. Create a
scatter-plot of the 1/V and P data and format it according
to your teacher’s instructions.
If the data do not turn out to be a nice straight line
on your graph then consult with your teacher immediately to
get help trouble-shooting.
Use the spreadsheet software to generate a line of best
fit or trendline. Be sure to show the equation and the
R-squared value on the graph. The slope of this line is
the constant of the proportion (k).
To the right of the Pressure column of data add another
column heading: “Pressure times Volume (kPa ×
Set up a calculation of pressure times volume. A likely
formula is “=A2*C2”. Copy the formula to all
the cells in the column. The result of this calculation
would be a constant for an inverse proportion.
At the bottom of your column of P × V values
calculate the average. Type:
Add yet another column to the right: “Pressure
divided by Volume (kPa/mL)”.
Set up a calculation of pressure divided by volume. A
likely formula is “=C2/A2”. Copy the formula to
all the cells in the column. The result of this calculation
would be a constant for an direct proportion.
At the bottom of your column of P ÷ V values
calculate the average. A formula you might use:
Format your data table for printing by making the
headings different from the cell contents and adding
Arrange to print each graph and the data table. Make
sure each lab group member will get a copy to hand in with
their answers to the post-lab questions. Students may hand
in the same graphs and table but must do their own work
in answering the questions.
Answer the following questions using complete sentences in
a professional-quality typed document. Submit them along with your spreadsheet with neatly labeled data and graphs.
What is the equation of the line for your second graph
in the form y = mx + b? Identify each of the letters in [y
= mx + b] with a quantity from the lab and clearly identify
the units of each quantity. Give decimal values for the
slope and y-intercept.
What is the average value of P × V? Is this
product a constant or nearly constant? Justify your answer.
What is the average value of P ÷ V? Is this
quotient a constant or nearly constant? Justify your
Check your data to answer the following questions:
What happens to the pressure when you double the
What happens to the pressure when you cut the
volume in half?
What happens to the pressure when you triple the
What happens to the pressure when you cut the
volume to one quarter?
What kind of proportion (direct or inverse) does your
first graph of P vs. V look like? What does your graph P
vs. 1/V tell you about the nature of the proportion between
P and V?
Based on your graphs and the answers to the previous
questions, should Boyle’s Law be written in
mathematical form as PV = k (an inverse proportion) or as
P/V = k (a direct proportion)? Justify your answer with
direct references to your graphs and calculations.
According to the introduction, is Boyle’s Law
valid if you change the temperature of the gas? What do you
think happens to the volume of a gas when you increase its
What do you think happens to the pressure of a gas when
you decrease its temperature? Think about the air pressure
in your car tires in the winter.