University of Pittsburgh at Bradford
Science In Motion
Chemistry Lab 010
MOLAR VOLUME OF A GAS
(SMALL-SCALE VERSION)
Introduction:
In chemistry many of the materials we work with are gases. It is often easier to measure the
volume of a sample of a gas than to determine its mass. A conversion factor is needed to convert volumes
of gases to moles of gases. In this experiment, known amounts of magnesium will be reacted with
hydrochloric acid to generate hydrogen gas. The moles of hydrogen and the volumes (at STP) will be
plotted. From the graph a value for molar volume will be determined.
Purpose:
The purpose of this experiment is to determine the volume of one mole of a gas at STP (molar volume)
Equipment/ Materials:
60 mL syringe
vial cap
LeurLOK syringe cap
ruler
magnesium ribbon
dilute HCl
30 mL beaker
Safety:


Goggles and a laboratory apron must be worn at all times for this experiment.
When removing the syringe cap, make sure that the syringe is not pointed at another individual.
Procedure:
1. Cut five pieces of magnesium ranging from 0.25 – 2.0 cm in length. Record the length of each in the
data table.
2. Directly weigh each piece of magnesium. Record mass of each in the data table (make sure you know
which piece is which). Place the first piece of magnesium in the vial cap.
3. Make sure the plunger moves smoothly in the barrel of the syringe. Remove the plunger, place your
finger over the tip of the syringe and fill it with tap water. Float the vial cap on the water in the
syringe and let the water drain into a waste beaker. When all the water has drained from the syringe
replace the plunger and push it into the syringe as far as it will go. Note the reading on the syringe.
You will use this value for the “cap volume” in Step 5.
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4. Draw 5 mL of the hydrochloric acid into the syringe. Place the LeurLOK syringe cap over the end of
the syringe and shake the syringe so that the acid and magnesium react.
5. When the reaction is completed, invert the syringe and remove the LeurLOK cap and drain the liquid
into the waste beaker. When the last drop of liquid is removed, replace the LeurLOK cap and read the
volume of gas in the syringe. Record the volume in the data table and subtract the “cap volume” in
Step 3 to obtain the actual volume of the gas.
6. Using the other pieces of magnesium, repeat steps 3 – 5.
7. Record the temperature and barometric pressure for the room in which you are working.
8. Plot the moles of hydrogen on the x-axis and the volume of the hydrogen (corrected, STP) on the Yaxis. Determine the slope of the line. (slope will be molar volume of H2)
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Name _________________________
Date __________________________
MOLAR VOLUME OF A GAS (SMALL-SCALE VERSION)
Student Evaluation
Data:
Room Temperature _________________________
Barometric Pressure ____________________
“Cap Space” from Step 3 _________________________
Trial /
Lentgh
Mass
Mg
Moles
Moles
Volume
Corrected
Volume
Mg
H2
H2
Vol. H2*
H2, STP
1/
2/
3/
4/
5/
* The corrected volume of H2 takes into account the “cap” space. Subtract cap space for Volume of H2
recorded in previous column.
**Note: highlighted columns are the values used to create graph in step 8 of procedure.
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Calculations:
1. For one of your trials, show the calculation of:
a. The mass of magnesium used
b. The moles of magnesium used
c. The moles of hydrogen formed
d. The volume of hydrogen (liters) collected at STP
e. The molar volume of hydrogen for that trial
Questions:
1. How did you determine the molar volume of hydrogen from your graph?
2. What are some possible sources of error for this experiment?
3. Draw a diagram that would show someone how to make conversions between a mass of a gas and the
volume of that gas (at STP).
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