Name
19
Date
_
Molar Volume of a Gas
PRE-LAB DISCUSSION
Avogadro's hypothesis states that equal volumes of all gases contain e,gual numbers of
molecules under the same conditions of temperature and pressure. It follows from this
hypothesis that all gas samples containing the same number of molecules will occupy the
same volume under the same conditions of temperature and pressure. A special name is
given to the volume occupied by I-mole samples of gases at STP. This volume is called the
molar volume. In this experiment, you will make an experimental determination of the
molar volume.
The basis of this experiment is the following reaction in which you will react a known
mass of magnesium with excess hydrochloric acid to produce the substances shown:
Mg(s) + 2HCI(aq)
'-"
->
MgClz(aq) + H2(g)
The hydrogen gas is the product that is of interest to you in this experiment. You will make
an experimental determination of the number of moles of hydrogen molecules produced
and the volume occupied by these molecules. The number of moles of hydrogen will be determined indirectly. The balanced equation for this reaction shows that the molar ratio of
magnesium reacted to hydrogen gas produced is 1:1. Therefore, by determining the mass of
magnesium that reacts and the number of moles that this mass is equal to, you will also
determine the number of moles of hydrogen gas produced. The volume of hydrogen gas
. produced will be measured directly on the scale of a gas-measuring tube. The gas laws of
Boyle and Charles will be used to correct this volume, measured under laboratory conditions, to the volume the sample of gas would occupy at STP. The collected data (number of
moles and volume at STP) will be used to calculate the molar volume of the hydrogen gas.
This experiment should aid in the understanding of the mole concept and the concept
of molar volume of a gas.
PURPOSE
Determine the volume of 1 mole of hydrogen gas at STP using experimental
known mathematical relationships, and a balanced chemical equation.
data,
EQUIPMENT
gas-measuring tube
one-hole stopper (for gas tube)
ring stand
utility clamp
thermometer
beaker, 400-mL
battery jar (or 1000-mL beaker)
graduated cylinder, lO-mL
metric ruler
safety glasses
MA TERIALS
magnesium ribbon (Mg)
3 M hydrochloric acid (HCI)
PROCEDURE
••
cotton thread
~
1. Obtain a piece of magnesium ribbon from your teacher. Measure the length of the piece of ribbon (± 0.1 cm). Record the
length as (a) in your data table. Also record the mass of 1 meter
of magnesium ribbon. These data will be provided by the teacher.
Record as (b).
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I
2. Obtain a piece of cotton thread about 15 cm long. Tie one end
of the thread around the piece of magnesium ribbon, leaving
about 10 cm of thread free. Bend the piece of magnesium so that
it will fit easily into the gas-measuring tube.
Obtain about 10 mL of 3 M hydrochloric acid (HCI). CAUTION: Handle this acid with care. Carefully pour the HCI into
a gas-measuring tube.
4. Tilt the gas-measuring tube slightly. Using a beaker, slowly fill
the gas-measuring tube with water at room temperature. Try to
avoid mixing the acid and water as much as possible.
5. Lower the piece of magnesium ribbon 4 or 5 cm into the gasmeasuring tube. Drape the thread over the edge of the tube and
insert the one-hole rubber stopper into the tube as shown in Figure
hole
stopper
'!I1-thread
coil of
Mg ribbon
gas tube
19-1.
6. Add about 300 mL of water at room temperature to a 400-mL
beaker. Set up a ring stand and utility clamp, and place the beaker of
water in the position shown in Figure 19-2.
7. Place your finger over the hole in the rubber stopper and invert the
gas-measuring tube. Lower the stoppered end of the tube into the
beaker of water. Clamp the tube in place so that the stoppered end is
a few centimeters above the bottom of the beaker (Figure 19-2).
Record your visual observations as (g) in the data table.
8. Let the apparatus stand about five minutes after the magnesium
has completely reacted. Then, tap the sides of the gas-measuring
tube to dislodge any gas bubbles that may have become attached
to the sides of the tube. Place your finger over the hole in the
stopper and transfer the tube to a battery jar filled with water.
Lower the end of the tube into the water and remove your finger
from the hole.
9. Move the tube up or down (to equalize pressure) until the water
level in the tube is the same as that in the battery jar. On the
scale of the gas-measuring tube, read the volume of the gases in
the tube. Record this volume as (c) in your data table.
10. In the data table, record the room temperature (d), and the
barometric pressure (e).
11. If time permits and your teacher so indicates, repeat the experimen 1.
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3M
HCI
Figure 19-1
Figure 19-2
Name
Date.
_
EXPERIMENT 19. MOLAR VOLUME OF A GAS (CONTINUED)
vg ofmm
2 Trials
em
em
'--./ volume
g°C
Hg Aof
mm
gTrial
visual
observations:
mL
mL
°C
room
temperature
H2
gasof
inMg
tube
water
length
vapor
ofof
pressure
ribbon
at2
barometric
pressure
mass
1;Mg
meter
(g)
mm
°C
em
gmL
PH20
Triall
page (from
204). Appendix B,
OBSERV A TIONS AND DATA
CALCULA TIONS
1. Find the mass of the Mg strip:
x grams
0.050 meter _
1.00 meter - mass of I-meter length
2. Calculate the number of moles of Mg reacted (which is equal to
the number of moles of H2 gas produced):
#
moles Mg
= mass of Mg reacted (g)
24 g Mg/mole Mg
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3. Find the pressure exerted by the
H2
gas in the tube:
4. Convert room temperature from DCto kelvin:
K=DC+273
5. Find the volume of the
H2
gas at STP:
where:
= PH, (from calculation 3)
= experimental value ofH2 (c)
= room temperature (K)
Tz =273 K
Pz =760 mm Hg
Vz= volume of H2 at STP
PI
VI
TI
6. Find the volume of 1 mole of Hz gas at STP:
moles of Hz gas (from calculation 2)
volume of Hz gas (V2)
x=
.mL/mole=
1 mole
xmL
L/mole
CONCLUSIONS AND QUESTIONS
1. The accepted value for the molar volume of a gas is 22.4 liters (22400 mL). How does your experimentally determined value compare with this accepted value? Calculate your percentage error.
2. What are some possible sources of error in this experiment?
3. Find the volume of the following masses of gases at STP:
(b) 10 g H2
(c) 14 g N2
(d) '66 g CO2
(a) 80 g O2
4. How many liters would the following number of moles of any gas occupy at STP?
(a) 0.25 mole
(b) 0.5 mole
(c) 1 mole
(d) 2 moles
(e) 2.5 moles
5. What happens to the other product of the reaction used in this experiment?
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