MHS Chemistry
Molar Volume of a Gas
The purpose of this lab is to determine the molar volume of hydrogen
gas by applying what knowledge of pressure and moles to an actual reaction.
Recall that "molar volume" means the volume occupied by one mole of something.
In this case, the "something" will be a gas under ideal conditions.
Procedure
-
Assemble the apparatus a shown by your instructor.
- Obtain about 5 cm of magnesium ribbon. Measure the mass exactly.
- Wrap the magnesium ribbon in a cage of copper wire. Leave a few inches
of copper hanging free for attachment.
-
Place 15 to 20 mL of 3.0 M HCl in the gas collection tube.
Gently top off the tube with distilled water.
-
Place the magnesium ribbon in the tube, hooking the copper wire from the
cage over the lip of the gas collection tube. Wedge the whole thing
together with a small two-hole stopper. Water should spill out of
the cork holes.
-
Cover the holes with your thumb, and invert the collection tube into the
large graduated cylinder of water. Clamp it in place, leaving room
between the tube and the bottom of the water. Observe the acid flowing
down inside the tube, and the formation of gas bubbles. Make sure
that ALL of the magnesium ribbon reacts. If some of it floats up
and sticks to the wall, slosh the tube around to keep the Mg submerged.
-
If some of the magnesium remains unreacted, repeat steps 1-6 with a little
less ribbon or a little more acid.
-
Raise or lower the tube so that the water level inside and outside are
the same. Record the exact volume of gas collected.
-
Record the room temperature (°C) and pressure (mmHg) from the back
of the room. Clean up your lab station, and put your goggles away.
Analysis
- Use the mass of magnesium to calculate the moles of magnesium used.
- Determine the pressure from water vapor in the hydrogen collected (the
gas is called “wet hydrogen” because it contains water vapor). This
pressure is only dependent on temperature. With temperature in °C,
the pressure of water vapor, in mmHg, is given as
P = 0.0334T2 – 0.243T + 8.9935
- Use the total pressure and the pressure from water vapor to determine the
partial pressure of hydrogen gas (this is the pressure from just the hydrogen).
Use the equation: P(total)
= P(water) + P(hydrogen)
- Use the relation P1V1 = P2V2
to determine the volume of pure (dry) hydrogen gas at 760 mmHg.
- Write the balanced equation for the reaction of magnesium metal with hydrogen
chloride.
- Use the moles of magnesium you started with to determine the moles of hydrogen
gas produced.
- Determine the volume of one mole of dry hydrogen gas at room temperature.
Give this volume in mL/mol and L/mol.
|
Data |
|
|
2. |
mass of Mg ribbon used |
__________ |
___ |
8. |
measured gas volume |
__________ |
___ |
9. |
room temperature |
__________ |
___ |
|
room pressure |
__________ |
mmHg |
|
Analysis |
|
|
a. |
Moles of magnesium in ribbon |
__________ |
___ |
b. |
Vapor pressure of water |
__________ |
___ |
c. |
Pressure of dry hydrogen |
__________ |
___ |
d. |
Volume of dry hydrogen |
__________ |
___ |
e. |
Balanced reaction
|
|
|
f. |
moles of hydrogen gas produced |
__________ |
___ |
g. |
molar volume of hydrogen gas @ room T |
__________ |
mL/mol |
|
|
__________ |
L/mol |
|
|
|
|
Shows all calculations in the space below.
Each person should turn in their own lab.
[Molar Volume of a Gas score sheet][MHS Chem page]