THE MOLAR VOLUME OF HYDROGEN GAS
INTRODUCTION AND THEORY
Molar volume is defined as the volume occupied by a mole of gas at specified conditions
like STP. Amedeo Avogadro postulated that 'at the same temperature and pressure,
equal volumes of all gases contain the same number of molecules'. The number of moles
of molecules of a gas in a sample of a given volume, at a given temperature and
pressure, does not depend on the identity of the gas.
Therefore, if we determine the volume of a known quantity of gas at a known temperature
and pressure, it is then possible to determine the volume at STP of this quantity of gas.
In this experiment, we will determine the molar volume of hydrogen gas. By reacting a
massed piece of magnesium metal with an excess of hydrochloric acid, hydrogen gas is
produced. The equation for the reaction is:
Mg(s) + 2HCl (aq) = MgCl2(aq) + H2(g)
Using a gas tube (eudiometer) we can determine the volume of hydrogen gas produced
at lab conditions. With the gas equation this can be converted to a volume at standard
conditions. If we know the volume of the number of moles of hydrogen gas we collected
at STP, we can calculate the volume of one mole (molar volume) at STP.
The hydrogen gas is collected by downward displacement of water. Consequently, at the
end of the experiment the gas in the tube will really be a mixture of hydrogen gas and
water vapor. We must then subtract the water vapor pressure from the atmospheric
pressure. (The atmospheric pressure equals the total pressure in the tube.) Water vapor
pressure is easily obtained from a table since it depends only on the temperature of the
water.
Patm
-
(barometer)
Pwater
(table)
=
PH2
(for calculation)
We will use this type of indirect measure in this lab. It is difficult to mass a small, light
piece of material; we can, however, mass a longer, heavier piece of the same material. If
we know accurately the lengths of both and the mass of the longer, we can calculate the
mass of the shorter.
PROCEDURE
1.
2.
Obtain a piece of magnesium ribbon a little less than 4 cm in length. If necessary,
cut a piece off a larger piece. Measure the ribbon carefully and record its length to
the nearest 0.01 cm (e.g. 3.92 cm).
Fold the magnesium up over itself several times. Wind a piece of fine copper wire
about this to form a cage. The cage should have no large openings through which
small pieces of magnesium ribbon can escape.
3.
Set up a ring stand and utility clamp in position to hold the 50 mL gas measuring
tube. Almost fill a 500 mL graduate cylinder with water which is at room
temperature. Stand the graduate under the utility clamp.
4.
Over the sink at the end of the bench incline the gas tube slightly from the upright
position. Carefully pour in about 10 mL of dilute (6.0M) hydrochloric acid, then
slowly fill it with distilled water. While pouring, rinse down any acid that may be on
the sides of the tube. Try to avoid stirring up the acid layer in the bottom of the
tube. Bubbles clinging to the side of the tube can be dislodged by tapping the tube
gently.
5.
Holding the copper cage by the handle, insert it into the tube a distance of about 10
cm. Hook the copper wire over the edge of the tube and hold it there by inserting
the rubber stopper. The tube should be completely filled with water such that water
must be displaced when the stopper is pushed in.
6.
With a finger on the rubber stopper, invert the tube and lower it into the graduate
cylinder. Clamp it in place. The acid, being more dense than water, will fall down
through the tube and eventually react with the magnesium.
7.
After the reaction stops, wait for about 5 minutes to allow the apparatus to come to
room temperature. Dislodge any bubbles clinging to the sides of the tube and the
copper wire.
8.
Raise or lower the tube until the level of the water inside the tube is the same as the
level of the water outside the tube. Read and record the gas volume to the nearest
0.01 mL. Without removing the tube from inside the graduate cylinder, use a
probe needle to force out the rubber stopper.
9.
Remove the gas measuring tube from the water and pour the acid solution it
contains down the sink. Rinse the tube with tap water. Record the room and water
temperatures. (These should be the same.) The barometric pressure will be written
on the chalkboard.
10.
Repeat the procedure above as directed by your instructor, at least one more trial
VAPOR PRESSURE OF WATER AT VARIOUS TEMPERATURES
TEMPERATURE (0C) PRESSURE (kPa)
20
2.3
21
2.5
22
2.6
23
2.8
24
3.0
25
3.2
26
3.4
27
3.6
28
3.8
29
4.0
30
4.2
DATA AND CALCULATIONS
Prepare and complete a table similar to the following
Atmospheric Pressure
= _________________ kPa
Mass of 100.0 cm of Mg(s)
= _________________ g
Room Temperature
= _________________ 0C
Trial One
Trial Two
Trial Three
Metal
Mass of Metal(s)
Partial pressure Of H2(g)
Volume of H2(g)
g
g
g
kPa
kPa
kPa
mL
mL
mL
Convert the volume of the H2 gas produced to a volume at STP for each trial. Calculate
the molar volume of H2(g) at STP for each trial. Average the volues for molar volume at
STP and calculate the percentage error in this average value.
CONCLUSION: Compare the experimental value for the molar volume of hydrogen to
the experimental value and attempt to account for any discrepancy.