Molar Volume of a Gas
By Maya Parks
Partners: Kelsea Floyd
3/18/15
Abstract:
This lab was performed to determine the molar volume of hydrogen gas at STP. We found the volume at
room temperature and pressure and used the combined gas law to find the volume at STP. We found
the molar volume of a gas to be 23.5 L/mol at STP. We were able to use concepts learned in class in this
experiment such as the combined gas law, as well as general understanding of stoichiometry. This lab
has 4.91% error, most likely due to inaccurate temperature values. This would not only change the
pressure of H2 gas, as we used temperature to find water vapor pressure, but also in the final calculation
of finding the volume at STP, we would have used the wrong temperature.
Purpose:
We will find the volume of one mole of H2 gas at STP using the combined gas law we learned in class.
This will allow us to demonstrate and employ our understanding of this important gas law, by providing
a real world example of a situation where the combined gas law would be helpful.
Materials:










Gas measuring tube (eudiometer)
250mL beaker
one-holed stopper
metric ruler
ring stand
thermometer
burette clamp
cotton thread or copper cage
magnesium ribbon
3M HCl
Procedure:
1.
Measure the length of a piece of magnesium ribbon. RECORD.
2.
Gently fold the magnesium and tie a thread around the piece of magnesium or place the
magnesium ribbon in the copper cage provided by your teacher
3.
RECORD mass of 1 meter of magnesium ribbon (on board).
4.
Invert eudiometer and add between 10-15 ml of 3M HCl
5.
Fill the eudiometer the rest of the way with distilled water
6.
Place magnesium on string in water solution
7.
Place holed stopper in open end of eudiometer
8.
Cover hole of stopper, invert eudiometer and place holed end in beaker filled ¾ of the way with
water
9.
Observe as the magnesium reacts with the hydrochloric acid. RECORD your observations.
10.
Let the apparatus stand for 5 minutes AFTER the magnesium as completely reacted. Tap the
sides of the tube to dislodge any gas bubbles.
11.
Measure the volume of the hydrogen gas in the eudiometer.
12.
Measure the height of the water tower in the eudiometer. The height is measured from the top
of the water in the beaker to the meniscus in the eudiometer.
13.
Measure the room temperature.
14.
Record the water vapor pressure and barometric pressure (on board or on TV screen).
15.
Repeat experiment two more times.
Data:
Length of magnesium ribbon
Volume of H2 gas
Height of water tower
Room temperature
Water vapor pressure
Observations
Trial 1
3.51cm
34.59mL
242.8mm
19.0C
16.477mmHg
Lots of bubbles, made the water
cloudy; water drops on side of
tube; air bubbles in water after
reaction finished
Trial 2
3.67cm
33.78mL
243.1mm
20.7C
18.310mmHg
Same as before; I also noticed
this time that the Mg ribbon
became black around the edges,
was less shiny during the
reaction, almost a powdery
white
Mass of 1m of Mg ribbon
Barometric pressure
.8825g
766.3mmHg
Calculations:
1.
Given the length of the Mg ribbon and the Mass of 1m of Mg ribbon, find the mass of the
magnesium.

Length of ribbon used, multiplied by the grams of a 1m long piece


2.
For trial 2, 0.324g
Calculate the number of moles of magnesium that reacted, using your answer from #1.

Divide by molar mass


For trial 2, .00133mol
3.
Calculate the number of moles of hydrogen produced using the answer from #2 and the
balanced chemical equation.

Multiply moles of magnesium by molar ratio


4.
00133moles of H2
Calculate the pressure inside the eudiometer



5.
Pinside = Pbarometric – ((height of water tower in mm)/13.6)
(
)
For trial 2, also 748.4mmHg
Calculate the pressure of the hydrogen only in the eudiometer:



PH2 = PInside – PH2O
For trial 2, 730.1mmHg
6.
Use the combined gas law to calculate the volume of H2 gas at Standard Temperature and
Pressure (STP).

Multiply pressure of gas by the volume and the standard temperature, and divide by the
temperature and the standard pressure

(

For trial 2, 30.2mL
7.
)(
(
)(
)(
)
)
Find the volume of 1 mole of H2 gas at STP, using your answers from #3 and #6.

Divide moles by volume


8.
For trial 2, 22.7L/mol
Calculate the average molar volume of hydrogen from the two trials

Add volumes and divide by 2

Error Analysis:
%error
During one trial, I accidently didn’t put the eudiometer in the water, and some bubbles came up into the
tube before the reaction began. This caused the molar volume to be greater, as it wasn’t just H2 gas; it
had other air in it.
Our thermometer was not very accurate, and it is possible that if the temperature was too high, it would
cause the volume to be less. We use the temperature to find the pressure of the water vapor which you
subtract from the overall pressure and results in a lower pressure of H2 gas, which is directly
proportional to the volume at STP in the equation. At the same time, this higher temperature is
inversely proportional to the volume at STP. Having the wrong temperature affects the final calculated
volume in two different ways, and a high temperature can cause the volume to be much less.
It would be much easier and wouldn’t change the results at all to use a larger beaker or jar. Putting the
tube in with your hand on the hole at the top of the stopper was difficult particularly as your hand
caused the water to be displaced and so you were never sure if the tube was far down enough, and if it
wasn’t, air could enter into the tube and mess up your results. Using a bigger jar or beaker would allow
more room to put your hand in, as well as have the water displacement be not so high as to mess with
putting the end of the tube in the water.
Conclusions:
1.
Why does the acid flow down the glass tube when it is inverted?

The acid is denser than the water, so it sinks down below the water. When the tube is inverted,
it naturally flows down the tube.
2.
In your lab, did you get a volume greater than or less than you would have gotten if you
collected your gas at STP. Explain in terms of room temperature and atmospheric pressure.

I got a volume greater than I would have at STP, as the temperature was much higher. The
pressure was slightly greater, about 6mmHg greater, but not enough to offset the increase in
temperature. Collecting the volume at 20°C caused the volume to be greater as the gas is more
excited and spread out.
3.
All real gases deviate to some extent from the behavior of ideal gases. At standard conditions,
the density of O2 gas is 0.0014290 g/mL, that of H2 gas is 0.00008988 g/mL, and that of CO2 is
0.0019769 g/mL.
Using these values and the molecular weights from your textbook (in g/mol, not rounded off), calculate
the molar volume of each of these, in mL/mol, to five significant figures.

O2

H2

CO2
Correlate the values of these three gases with the molar volume of an ideal gas (22.4L/mol).
Which gas deviates least?
H2
Which gas deviates most?
CO2
Suggest a reason for this behavior.
From the deviation, it seems as if molecules with higher molar masses deviate the most. Solutions with
heavier molecules are denser, even as gases, and show this with smaller volumes per mole. Lighter
molecules are not as dense and are much closer to 22.4 L/mol.