Experiment 13: Gas Laws
Prelab–Introduction
Is there a constant relationship between the pressure of a gas and its amount, its temperature and the
volume of its container? The pressure of a gas is a measure of how strongly the molecules or atoms of
the gas are colliding with the surface of the container it is stored in.
Read the experimental details page, then answer the following questions:
1. Write balanced chemical reactions for the reactions of HCl(aq) with Mg(s), and for the decomposition of
hydrogen peroxide [H2O2(aq)] with a catalyst.
2. If a gas is in a sealed, rigid container, and the temperature is increased, the pressure will:
a) increase
b) decrease
c) stay the same
3. If a gas is in an expandable container, and the volume of the container is increased without allowing
the temperature to change, the pressure will:
a) increase
b) decrease
c) stay the same
4. If a gas is in a sealed container, and more gas is added at constant temperature, the pressure will:
a) increase
b) decrease
c) stay the same
5. Using your answers above, fill in the following blanks:
The pressure of a gas is directly dependent on its __________________ and its
__________________, but inversely dependent on the _____________________.
6. Fill in the equation based on your answers above
(use P for pressure, T for temp, V for volume and n for number of moles, ∝ means proportional)
P ∝
7. Write balanced chemical equations for the decomposition of H2O2 (O2 is the only gas formed) and for
the reaction of Mg(s) with HCl(aq).
You and your lab partner will prepare and collect two gases to determine if there is a relationship between
the pressure, volume, number of moles and temperature of 2 gases.
PV
You will be looking to see if the ratio of nT is a constant, (C), regardless of the gas.
C=
PV
nT
where P = pressure (in mmHg) V = volume (in mL)
n = number of moles T = temp (in K)
In this lab, will you will determine if ‘C’ is constant for measured amounts of H2 and O2 gas.
Experimental Details:
To calculate C, pressure, volume, number of moles and temperature of the gas must be measured.
In today’s experiment, oxygen will be prepared by decomposing hydrogen peroxide (H2O2) with one
of a number of catalysts, like KI, FeCl3, Mn(s) or yeast. The amount of oxygen produced will be
determined by measuring the mass or volume of water displaced as the gas pushes water out of a
partially sealed system. Hydrogen gas will be generated by reacting Mg with HCl. The volume of
the H2 gas will be measured in a device called a Eudiometer tube, which might be Greek for “really
long test tube.”
Here are details of how the required values will be measured:
Temperature, T: The temperature will be determined by measuring the temperature of the water below
the gas. The gas should be approximately the same temperature as the water.
Volume, V: The O2 will be pushed out of a partially sealed system, and the volume of water displaced
or the mass (and thus volume via density) will be measured. For H2, the eudiometer tube has
graduations on it, so you will be able to directly measure the volume of H2 gas that is formed.
number of moles, n: In the first reaction, the mass of the H2O2 lost will be measured by the mass lost.
In the second reaction, all of the Mg will react and HCl will be in excess. The number of moles of gas
formed can be calculated from the amount of the limiting reactant (Mg or H2O2) using stoichiometry.
Balanced chemical equations are necessary.
Pressure, P: Pressure will be a little more difficult to determine, but only for H2. There are three
things to consider when determining the pressure exerted by the H2 gas in the Eudiometer tube:
barometric (or atmospheric) pressure, liquid level difference and the vapor pressure of water.
Barometric pressure: The pressure of the atmosphere will be the same as the pressure in the
Eudiometer tube, and you will measure this with a barometer. Read this section in your text.
Level difference: The fluid level in the Eudiometer tube must be equal to the water level in the outer
container, or the internal pressure will not be the same as the barometric pressure. If the water level
outside the tube is greater than that on the inside, the pressure inside will be greater than the outside
pressure, causing the volume to be smaller than if the pressures were equal.
Vapor pressure of water: Some of the water in the eudiometer tube will evaporate. This means that
the amount of gas in the eudiometer tube is equal to the gas produced in the reaction (H2 or O2) plus
the H2O gas that has evaporated. Each gas exerts a pressure inside the tube. The amount of water
that will evaporate (and hence the pressure due to gaseous water) depends on the temperature. The
pressure exerted by this gas is called the vapor pressure of water. The vapor pressure of water at
various temperatures is given your text. To determine the Cuesta Constant, we need to use the
pressure due to the gas produced in the reaction only. The total pressure of all gases in the tube is
equal to the sum of the pressure of each gas, so
Reaction 1: Ptotal = PO2 + PH2O
Reaction 2: Ptotal = PH2 + PH2O
Once you are done with all of these measurements, calculate the constant in units of mL·mmHg/mol·K.
Safety Precautions
• H2 is highly flammable. There should be no open flames or electrical sparks in the lab.
•
Be very careful adding the HCl to the eudiometer tube. Report any acid spills to your instructor
for help cleaning it up.
•
Do not let the Mg ribbon fall to the bottom of the tube before you invert the tube. If your Mg
metal begins to fall to the bottom, quickly invert the tube over the sink and allow to drain.
•
All waste can go down the drain, except excess 6 M HCl, which goes in a waste container.
Procedure
Preparation of O2.
1. Set up the apparatus as demonstrated at the front bench.
2. Measure the mass of the displaced water beaker C, if appropriate balances are available (the
normal lab balances cannot measure the mass of water and the beaker).
3. Add about 60 mL of 3% hydrogen peroxide to vessel A.
4. Fill the bottom of a plastic vial cap with a single layer of solid catalyst.
5. Record the combined mass of flask A, its contents, and the vial cap + yeast. Using forceps,
carefully lower the cap full of catalyst into vessel A.
6. Assemble the apparatus as demonstrated at the front bench, leaving vessel A separate. Fill the
oxygen collection flask (B) nearly full with tap H2O and the 400 mL displaced water beaker (C)
with about 150 mL of tap water.
Pinch clamp
tube 1
tube 2
Reaction vessel A
H2O2 and catalyst
Oxygen collection
flask B
Displaced water
beaker C
7. Disconnect the rubber stopper from vessel A. With the pinch clamp open, use a pipet bulb to blow
air into tube 1 by squeezing the pipette bulb. Keep the end of tube 2 below the water level in
beaker C. This will force water from flask B into beaker C, and fill tube 2 with water. Beaker C
can be raised and lowered, or suction can be applied with the pipet bulb so that no air bubbles
remain in tube 2.
8. Connect the stopper at the end of tube 1 to vessel A and seal tightly. DO NOT allow the catalyst to
mix with the hydrogen peroxide in vessel A.
9. Raise beaker C so that the level of water in beaker C is equal to the level of water in flask B. This
will make the pressure equal in all three flasks, which is also equal to atmospheric pressure. While
the water levels are equal, have your lab partner close the pinch clamp tightly.
10. Carefully lower beaker C and remove tube 2 from the beaker so as to not lose the siphon. If the
water all leaks out of tube 2, start the leveling procedure all over again. Pour out all of the water in
the beaker C, dry the beaker, and return tube 2 so it’s inside the beaker.
11. Open the pinch clamp. If done correctly, a little water will dribble into beaker C. However, the
water flow should cease almost immediately. If it does not, there is a leak in the system. If this is
the case, check to make sure all the connections are tight and re-level the system.
12. After opening the pinch clamp, gently mix vessel A so that the catalyst spills out of the test tube
into the hydrogen peroxide solution (avoid getting the solution on the stopper). Swirl the flask to
mix the reactants. Observe what happens in vessel A, flask B, and beaker C. Keep swirling until
the reaction appears to be complete and water stops flowing.
13. Raise beaker C until the water levels in flask B and beaker C are equal, and have your lab partner
close the pinch clamp on tube 2. Measure the mass of beaker C to determine the mass of the water
displaced. If appropriate balances aren’t available, the volume of oxygen produced can be
measured by carefully pouring the water in beaker C into a graduated cylinder. Record the either
the mass or the volume of water displaced.
14. Record the mass of the contents of vessel A to determine the mass of the hydrogen peroxide that
has decomposed.
15. Obtain the barometric pressure from your instructor.
16. Measure the temperature of the water remaining in flask B. This will represent the temperature of
the gas. An electronic temperature probe may be available, or use a thermometer.
17. The waste can be washed down the sink.
Preparation of H2.
18. To prepare and collect H2 gas: Clean a 2-3 inch piece of Mg ribbon. Weigh a single, 0.05 to 0.09
g piece of Mg ribbon on a top loading balance to the nearest 0.0001 g. Do NOT use more than
0.0900 g
19. Obtain about 40 mL of 6 M HCl in your graduated cylinder. Add approximately 10-15 mL of
6 M HCl to the eudiometer tube. With your wash bottle, wash into the tube any HCl that is on the
walls of the tube. Continue to fill the tube to within 3 inches of the top with water, making an
effort to reduce turbulence and mixing of the water and HCl.
20. Take a 3 inch piece of Cu wire and make a 180° bend 1 inch from the bottom. Loosely coil up the
Mg (not too tight) and place over the bend in the copper wire, bending the Cu to secure the Mg
ribbon coil. Insert the bare end of the Cu wire through the small end of the stopper, and plug the
Eudiometer tube. The Mg coil should be about 1 inch inside the Eudiometer tube.
21. Bend the wire over the lip of the eudiometer tube, and fill any remaining space in the Eudiometer
tube and stopper hole with water. Fill a large graduated cylinder with water, and place in an
overflow container.
22. Cover the hole in the stopper with your thumb. Invert the tube into a large graduated cylinder in an
overflow trough, and clamp the tube to secure. Record the initial volume gas in the eudiometer.
23. After the reaction is complete, allow it to stand for 3 minutes. Record the temperature of the water.
Making sure there is NO difference in the levels of liquids in the outer container and eudiometer
tube, record the final eudiometer volume. If the Eudiometer tube wont’ go low enough, consult
your instructor.
24. When you are finished, rinse the eudiometer tube with water (it does not need to be dried) and start
a second trial. The liquid in the cylinder (or beaker) can go into the sink, but also turn on the water
for a while to dilute the acid. While trial #2 is reacting, complete the calculations for trial #1.
Perform the experiment a total of three times.
Gases Report Sheet
Name _______________________________
Partner ______________________________
Table 1: Decomposition of H2O2 via a Catalyst
Mass of vessel A and its contents before reaction
Mass of vessel A and its contents after reaction
 Mass of H2O2
 Moles of H2O2
 Moles of O2 (from moles H2O2)
Temperature of the water (and gas) in flask B (K)
Mass of dry beaker C
Mass of beaker C after O2 collection
 Mass of water collected (g)
 Volume of water (and oxygen gas) collected (mL)
Barometric pressure (inches Hg)
 Barometric pressure (mm Hg)
Water vapor pressure in flask B
(see Appendix 1, interpolate the value)
 Pressure of O2 gas (mm Hg)
Attach a separate sheet neatly showing a calculation for each starred () result in the table above.
1. Calculation and result for the gas constant with data above, in units of mL·mmHg/mol·K
_____________________
Table 1: H2 Prepared from Mg and HCl
Trial 1
Trial 2
Trial 3
Mass of Mg (g)
____________
____________ ____________
Moles of Mg)
____________
____________ ____________
Initial eudiometer reading (mL)
____________
____________ ____________
Final eudiometer reading (mL)
____________
____________ ____________
Temperature (in K)
____________
____________ ____________
Barometric Pressure (mmHg) _____________
Vapor pressure of water at T
___________
corrected pressure of H2 _____________
2. Calculate the constant (units are mL·mmHg/mol·K) for hydrogen gas for your 3 trials. Show the
calculation for one trial. Consider how many significant figures should be included.
gas constant (mL·mmHg/mol·K)
__________
__________
__________
3. Calculate the mean for the gas constant for H2. Always include units.
4. Calculate the percent difference for each trial from the mean (show one calculation, also called
percent error). Does this tell you if your data is accurate, or precise?
____________
____________ ____________
5. Calculate the percent difference in C from the two methods (100% × difference ÷ average).
6. Based on your answer to number 5, is the gas constant really a constant? Briefly explain your
reasoning.
7. Convert the average value for the gas constant for H2 to units of L•atm/mol•K.
8. Calculate the percent difference between the gas constant for H2 and the Universal Gas Constant.
9. Compare the precision and accuracy of each method for determining the gas constant. Use specific
citations of your data.
10. If 0.109 g of aluminum metal reacted with excess HCl in an experiment conducted in Denver, CO,
calculate the theoretical volume of H2 that would be collected over water if the temperature of the
gas was 19.0 °C and the atmospheric pressure was 29.32 inches of Hg.
11. Describe how your calculated value of the gas constant would be affected if a piece of Mg that was
used was impure, and the impurity did not react with HCl to make a gas.
12. Potassium chlorate can be decomposed to O2 and potassium chloride by heating. Calculate the
volume of O2 produced, measured at STP, from the decomposition of 12.3 g of potassium chlorate.