CP Chemistry
Theodore Roosevelt High School
Lab #3-5
Ideal Gas Constant Lab
Introduction
The ideal gas law is represented by the equation PV = nRT, where R is the ideal gas constant. In this
lab, you will attempt to experimentally determine the value of R. To do this, you must determine the
values of P, V, n and T by generating and collecting a sample of hydrogen gas from the reaction
between magnesium and hydrochloric acid in an upside-down graduated cylinder. The hydrochloric
acid will be in excess and the magnesium ribbon will be wrapped in a copper wire ‘cage’ to ensure that
the magnesium ribbon reacts completely and produces the proper amount of hydrogen gas.
Purpose
Based on the introduction above, determine the purpose of this lab and include it in your lab write-up.
Prediction
Do you expect that the atmospheric pressure in the lab will be above or below standard atmospheric
pressure (101.3 kPa)? Explain.
Materials
copper wire [Cu]
hydrochloric acid, 3.0 M [HCl]
magnesium ribbon [Mg]
Equipment
beaker, 400 mL
gas pressure sensor
graduated cylinder, 10 mL
latex gloves
pipette
rubber stopper, one-hole
ruler
thermometer
Safety Considerations
• 3M hydrochloric acid is dangerously caustic! Avoid contact with the
skin and eyes.
• Safety goggles must be worn at all times; gloves must be worn in
Steps #5-8.
• Sometimes chemicals from previous labs still remain in glassware and
on other lab equipment; wash all lab equipment before and after
performing this lab.
• Wash your hands thoroughly after completing this lab.
Procedure
1.
Using a ruler, measure and record the exact length of a small
piece of magnesium ribbon. The length of the ribbon piece
should be between 0.6 and 0.8 cm.
2.
Wrap the copper wire around the magnesium ribbon,
making a ‘cage’ that surrounds the ribbon as shown in the
first figure to the right. Leave a handle of copper wire
approximately 6 cm long.
3.
Insert the handle end of the copper wire into the one-hole
rubber stopper as shown in the second figure to the right.
4.
Fill the 400 mL beaker approximately half full with tap water.
5.
While wearing latex gloves, use a pipette to add
approximately 3 mL of 3.0 M hydrochloric acid to the
graduated cylinder.
6.
Using the pipette, gently fill the graduated cylinder by
drizzling water down the inside wall of the cylinder to avoid
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Theodore Roosevelt High School
Lab #3-5
mixing with the acid. Since HCl is more dense than water, it will stay at the bottom of the cylinder.
Gently insert the stopper into the graduated cylinder while keeping the copper wire cage at the top
of the cylinder.
While holding your finger over the hole in the rubber stopper, quickly but carefully turn the
graduated cylinder upside down and place it into the beaker of water as shown in the third figure
to the right. Once the top of the cylinder is underwater, remove your finger and rest the cylinder in
the beaker while the reaction proceeds.
When the magnesium ribbon is no longer reacting, tap the side of the cylinder to release any
trapped gas bubbles.
Let the cylinder sit for 5 minutes so that the temperature of the system returns to room
temperature. Measure and record the temperature of the water in the beaker.
Using the gas pressure sensor, measure and record the atmospheric pressure in the lab.
Lift the graduated cylinder slightly until the levels of water inside and outside the cylinder are the
same. Measure and record the volume of gas in the cylinder.
Remove the cylinder from the beaker, remove the stopper from the cylinder, and dispose of the
liquid in both containers in the sink. Clean all lab equipment used and return each item to its
proper place.
Additional Clean-up and Disposal
1.
Empty the solids from your waste beaker in the trash and dump the tap water in the sink.
Data – you should create a data table in your lab write-up that looks something like this:
Property
Value
length of Mg ribbon
water temperature
Water Vapor Pressure Table
T (°C)
0
1
2
3
4
5
6
7
8
9
10
11
P (kPa)
0.61
0.66
0.71
0.76
0.81
0.87
0.93
1.00
1.07
1.15
1.23
1.31
T (°C)
12
13
14
15
16
17
18
19
20
21
22
23
P (kPa)
1.40
1.50
1.60
1.70
1.82
1.94
2.06
2.20
2.34
2.49
2.64
2.81
T (°C)
24
25
26
27
28
29
30
31
32
33
34
35
P (kPa)
2.98
3.17
3.36
3.56
3.78
4.00
4.24
4.49
4.75
5.03
5.32
5.62
T (°C)
36
37
38
39
40
50
60
70
80
90
100
P (kPa)
5.94
6.27
6.62
6.99
7.37
12.3
19.9
31.2
47.3
70.1
101.3
Calculations (Include these answers in your Conclusion!)
1.
Knowing that 100.0 cm of magnesium ribbon has a mass of 1.06 g, calculate the number of
moles of magnesium that were reacted (hint: convert your recorded length to grams, then to
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Theodore Roosevelt High School
Lab #3-5
moles).
Using the balanced equation for the reaction between magnesium and hydrochloric acid,
determine the number of moles of hydrogen gas that were produced.
Determine the pressure of the hydrogen gas by subtracting the water vapor pressure of the
system from the atmospheric pressure.
Convert the volume of hydrogen gas produced from milliliters into Liters.
Convert the temperature of the system into Kelvins.
Using the pressure, volume, temperature and moles of hydrogen gas in this system, calculate
your experimental value of the ideal gas constant, R.
Using the accepted value for the ideal gas constant, R, determine the percent error for your
experimental value of the ideal gas constant.
Questions
1.
Using your experimental value of R, determine the molar volume of a gas at STP (solve for n if V
= 1 L). How does this compare to the accepted value, 22.4 L/mol?
2.
Describe three observations from your lab that showed a chemical reaction took place.
3.
When exposed to hydrochloric acid, the copper wire reacted very differently than the magnesium
ribbon. What chemical explanation can you give for why this occurred?
Errors
Describe two possible errors you may have committed in this lab that may have somehow affected your
results. Explain the specific steps you will take to avoid each of these errors in the future.
Conclusion
Write two or more paragraphs summarizing your results, examining the validity of your prediction, and
explaining your calculations.
CP Chemistry
Theodore Roosevelt High School
Ideal Gas Constant Lab workspace:
Lab #3-5