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Mr.D©2017
Lab: Determination of Molar Amount of Gas Using C.G.L.
Purpose:
The purpose of this lab activity is to use the combined gas law in conjunction with
Avogadro's law to determine the molar amount of hydrogen gas collected from a
reaction between hydrochloric acid and magnesium metal. Dalton’s Law of Partial
Pressures will also be used to correct determine the pressure of the hydrogen gas
collected as part of a mixture through water displacement. To conclude the analysis
you will use your previous knowledge of stoichiometry to determine the mass of each
reactant used in the reaction based on the molar amount of gas collected.
Materials:
 100 ml graduated cylinder & 50 ml eudiometer
 10 ml graduated cylinder & large tub
 3 Large test tubes, test tube rack and stoppers
 One strip of magnesium approximately 2.50 cm & 15 – 20 ml of hydrochloric acid
Important Conversions:
In the United States we do not use the metric system. You will be given the
barometric pressure in common English units of inches of mercury and you will have
to correctly convert to a useable pressure unit for this lab.
1 atm = 29.92 in Hg
1 meter Mg = 1.6818 grams
1 Mole of gas at STP = 22.4 L
Mr. D© 2004
Pre-lab Questions: (answer on separate sheet of paper and attach to lab at end)
1. What are the units for S.T.P.?
2. How many moles of sulfur trioxide gas are in a sample that has a volume of 36.35 liters it the
sample is at S.T.P.?
3. If there was 40.51 liters of sulfur trioxide gas at 27°C and 1.12 atm, find the volume at S.T.P.
4. Write the balanced Chemical Equation for the reaction in this lab.
Procedure:
Setting up your gas collectors:
1. Obtain all the items from the materials list for your station. If anything is
missing see the teacher.
2. Send one person to get a sample of magnesium metal (measure and record
its length in cm to the .01 cm) and another with a small graduated cylinder
to get the 15-20 ml of the hydrochloric acid. (When you measure the
volume make sure you are using the bottom of the meniscus to read the
volume on the graduated cylinder).
3.
Fill the tub to about 60% capacity with tap water and set aside.
4. Next, take the 100 ml graduated cylinder and fill it to the very top with tap
water until it is just overflowing…invert it in the tub as instructed by your
teacher and make sure there is no air in the cylinder.
5. Fill and invert the eudiometer and two test tubes in the same manner. All
four vessels will need to be filled with hydrogen gas
Setting up your gas generator:
1. Place your magnesium strip in your gas generator and place the stopper
back on.
2. Add the acid to the generator through the thistle funnel and once the air has
left the tube, place it under your gas collector vessel and collect the
hydrogen gas produced from the reaction.
3. Once the gas production stops, record the volume of hydrogen collected to
the correct degree of accuracy as well as the difference between the water
height in the gas collector and the water in the tub.
Repeat this process for the eudiometer and the graduated cylinder
Test Tube Filling and Splint Tests:
1. Set up your gas generator one more time and fill two of the test tubes with
hydrogen and then place a rubber stopper on the end to keep the hydrogen
from leaking out.
2. Remove the stopper from one of the test tubes and immediately place a
burning splint at the opening…record the result
3. Take the tube from step 2 and invert it over top of the second tube filled
with Hydrogen and wait 1 minute. Test by lifting the test tubes
approximately 1 cm apart and inserting a burning splint between the two
test tube openings. Record the results and compare to the observation in
step 2.
At this point clean up your area, place all the equipment back where you found
it and make sure you have all your observations and necessary data recorded
and shared with your group members. This will be a good time to also assign
who will be responsible for each portion of the full lab report.
Mr. D © 2004
Analysis:
1. Explain what you observed and the cause of any difference between the results of the
test tube splint tests.
2. What device would be give you a more accurate result for the moles of hydrogen gas
collected? Explain what makes the eudiometer a more/less accurate device?
Calculations:
1. Calculate the mass of magnesium you used in this lab.
2. Show your calculation for the pressure of the hydrogen collected in each device.
3. Calculate the volume each of your collected samples of hydrogen would have at STP
4. Calculate the moles of hydrogen gas collected in each vessel in this lab.
5. Calculate the theoretical hydrogen that should have been produced in this lab
6. Calculate the % error and % yield based on hydrogen.