Gay-Lussac’s Law Relating Temperature and Pressure of Gases
Introduction
Background topics should include: Gas properties and behaviors, ideal vs. real gases, and at
least one paragraph each on Boyle and his law, Gay-Lussac and his law, and Charles and his law.
When discussing each person and law, you should be sure to fully explain WHY the variables
behave as they do (speed, kinetic energy, collisions, etc.)
Objective(s): to demonstrate Gay-Lussac’s law that pressure and temperature are directly related.
Hypothesis: As temperature is increased, what will happen to the pressure of the air trapped in a
mason jar? Why in terms of average kinetic energy/speed and collisions?
Materials and Methods
1. Obtain a gas laws apparatus as shown in figure 1 below. Make sure that stopcock A and
stopcock B are both closed. Closed means that the blue
lever is parallel to your lab table or going right to left as
opposed to up and down.
2.
Fill a 60 mL syringe with air by pulling back on
the piston until the bottom end of the rubber stopper
reaches the 60mL mark.
3.
Connect the FULL 60 mL syringe to stopcock B,
“Injection Port,” while it is closed. To connect the
syringe, you must place it on top and the carefully/gently
screw it on.
4.
Now open stopcock B, “Injection Port”.
5.
GENTLY, push the piston of the 60 mL syringe
into the barrel forcing the gas into the Mason jar. When all of the gas in the syringe has
transferred into the jar close stopcock B, “Injection Port”.
6.
Open and immediately close the other valve, stopcock A, “Pressure Port”, the pressure
reading should be somewhere between 50-70 mm Hg.
7.
Turn on the Vernier interface and set up the thermometer probe as directed.
8.
Fill a 1000 mL Beaker with 300 mL of cold tap water to create a water bath for the gas kit.
9.
Also prepare a hot plate by plugging it in and putting it in setting #5. Take care not to burn
yourself. Also be aware of the hot plate and the Vernier temperature wire at all times! DO
NOT ALLOW IT TO GET BURNED/MELT! Or you may have to pay $$$$$!!!!
10. Immerse the gas kit in the cold water mixture in the 1000 mL beaker. Put someone in charge
of holding the mason jar this whole time! Allow it to settle into the cool water for a few
minutes and hopefully the thermometer will soon read ~20.0°C.
11. When the temperature is about 20.0oC, record the exact temperature in Table #1 below and
carefully open stopcock A, “Pressure Port” count very slowly to 3, and close stopcock A,
“Pressure Port”. Gently tap the pressure gauge with your finger. Read and record the gauge
pressure to the nearest mm Hg (the ones place will be estimated). Always open and close
stopcock A, “Pressure Port” and tap the pressure gauge in this manner to obtain the pressure
of the gas in the jar. Record the gauge pressures in Table #1.
12. Now carefully move the water bath and entire gas apparatus so that it can placed on top of
the hot plate that has been on and heating up. Use an adjustable clamp and ring stand to
connect to and keep the mason jar properly submerged. Adjust the clamp so that only the
glass part of the Mason jar is covered by the ice water mixture. You must be very careful
during this step now to allow the jar to pop out and break and also to not allow the clamp to
loosen anything and let the gas seep out.
13. Diligently monitor the temperature of the mason jar full of air. Get pressure readings every
time the temperature increases by 5°C, so at 25.0°C, 30.0°C, 35.0°C, etc. up until 75.0°C if
you can get there in time. Record temperatures and gauge pressures in Table 1. To get the
pressure reading, remember that every time you are ready to get the pressure reading, you
must open stopcock A, “Pressure Port” count very slowly to 3, and close stopcock A,
“Pressure Port”. Gently tap the pressure gauge with your finger. Read and record the gauge
pressure to the nearest mmHg. Always open and close stopcock A, “Pressure Port” and tap
the pressure gauge in this manner to obtain the pressure of the gas in the jar.
14. Turn off the hot plate once the temperature of the gas exceeds 75oC or if time runs out and
the instructor says to do so. Don’t let the water boil!
15. Clean up according to instructor’s directions. Don’t touch anything hot without permission
and protective oven mitts.
16. Immediately after cleaning up, review the procedure above to make notes and edits to
account for the exact procedure YOUR GROUP followed. You will need to have these
edits/specifics in your lab report.
17. If time permits in lab, go to Table #1 and start working with your data. The actual pressure
of the gas in the Mason Jar is Atmospheric Pressure + Gauge Pressure. While atmospheric
pressure varies daily by location, it should be relatively close to STP so just use 760 mmHg
as the reading for atmospheric pressure.
pactual = patmospheric + pgauge
In your lab reports, you don’t need to show these above as official calculations in a
calculations step, but all need to be done and recorded in Table 1. ALSO – be sure
to include an explanation of this step in your methods to explain how actual
pressure was determined.
Also convert all temperatures from °C to K. No need to show calculations for this,
but put values in Table 1 and explain in methods briefly how this was done.
18. For lab reports, use graph paper to plot a graph to compare the temperature of the gas
with the pressure of the gas. The scale on the temperature axis should be in Kelvins and to
avoid a “squished set of data,” it is ok to have 0K and then a “break” and then start with
280 K. Be sure that you have an even temperature scale and an even pressure scale even
though you are plotting your actual values. The same thing is true for the pressure scale.
Since mmHg is such a large unit, you are certainly allowed to have a “break” after 0
mmHg and begin your pressures at 760 mmHg (atmospheric pressure). See below for
more details about this graph.
Results
Table #1 – Pressure and Temperature Data
Temp (°C)
Temp (K)
Gauge Pressure
(mmHg)
Atmospheric
Pressure (mmHg)
Actual
Pressure (mmHg)
Report should include Table #1 above recreated in report.
Report must also include a handmade graph of Actual Gas Pressure (mmHg) vs. Temperature
(K). Connect these points as a best fit line. Research how to do a best fit line if needed.
Calculations:
Use 30.0°C data as your P1T1 and determine mathematically/theoretically what the P2 was
supposed to be at 50.0°C. (I have chosen this temperature randomly so you don’t have to do this
for all of them.) Show your calculation for P2 at 50.0°C by ignoring the experimental value for P
at 50.0°C. Show this in the 3 step method and use proper sig figs, etc.
Now using the above calculation as your theoretical P2 at 50.0°C, perform a percent error
calculation for theoretical P at 50.0°C comparing that to the experimental value for P2 at 50.0 °C
from Table 1. *** Be sure you have used Kelvins!!
So in your report – you will have only 2 official calculations – one for calculating the theoretical
P2 at 50.0°C and one for percent error at 50.0°C for P.
Discussion
Hints for what to discuss…

Infer about trend found on Figure 1 and if it upholds Gay-Lussac’s Law.

Explain what was happening in the experiment to the air molecules throughout in terms
or molecule speed, collisions, etc.

Comment on accuracy of experiment by discussion calculations done and making
inferences.

Include an extensive error discussion of all possible sources and HOW they could have
affected the data.

Be sure to note if hypothesis was correct and if/how objectives were met. (Note that this
does NOT have to be the last paragraph. By all means, work it in earlier if possible.)
References
APA as usual – continue to properly use my quick reference guide.