LABORATORY EXERCISE
Date Submitted: __________________________
Team members:
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The Determination of the Value for Molar Volume
Objective/Goal: Using a chemical reaction that produces a gas, measure the appropriate values to allow a
determination of the value for molar volume. (How many liters of dry hydrogen gas at room temperature
and pressure can be produced per mole of magnesium metal.
Brief Overview: The chemical reaction to be used is this: Mg(s) + 2 HCl(aq) ---> MgCl2(aq) + H2(g)
This reaction has a 1:1 mole ratio between magnesium used and hydrogen produced. This will allow an easy
determination of the moles of H2 by determining the mass of Mg consumed in the reaction. The mass of
Mg will be determined by comparing the length of ribbon used to the mass of one meter of ribbon. A
hidden assumption of this lab is that two different lengths of the ribbon are uniform in width & thickness.
The gas will be collected "over water." This means that the gas produced is in contact with liquid
water. The consequence of this is that the gas in the tube WILL NOT be pure H 2; it will have some water
vapor mixed in. The mixture is usually called "wet gas" and its production is unavoidable. However, using
Dalton's Law of Partial Pressures, we will be able to remove the water vapor's pressure, thereby
recovering the "dry" hydrogen gas' pressure.
Corrections to the temperature and pressure at the time of the experiment will also be required,
since the molar volume value we are seeking is measured at Standard Temperature and Pressure (STP).
Using the Combined Gas Law, we will be able to make the necessary corrections.
Safety Reminder: Wear goggles during the entire lab, including setting up and cleaning up. You will be
asked to leave the laboratory THE VERY FIRST TIME you do not comply with this safety procedure. The
hydrochloric acid (3M HCl) we will use can HURT you. Use it with great care.
Materials List:
Equipment
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50 mL gas measuring tube
30 cm piece of copper wire
1-hole stopper
150 mL beaker
600 mL beaker full of water
500 ml graduated cylinder
centimeter ruler
water bottle
pieces of chalk
Reagents
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small piece (≈5 cm) of magnesium ribbon
15 mL of 3 molar (3 mols/ml) HCl
Procedure
1) Fill the 600 mL beaker 3/4 full with tap water.
2) Measure the Mg ribbon's length with the ruler. Measure it and record
this value in your data table. Your teacher will give you the mass of 1 m
of the Mg ribbon.
3) Fold the Mg ribbon over twice, then create the copper wire cage around
it. Wind the wire in different directions, so there are many small
openings rather than one or two big ones. Leave 8-10 cm or so of wire
unrolled to act as a handle so that the cage can be put up to
approximately the 50-ml mark in the gas measuring tube.
4) Set up a ring stand and utility clamp in position to hold the gas collecting
tube.
5) Fill the gas measuring tube with approximately 15 mL of acid at the
front desk, then return to your work area. (The HCl is in excess in this
experiment, so don’t fuss about the exact volume of HCl) Those not
carrying the tube with acid must yield the right of way to those who are.
This is an extra precaution when using concentrated acid).
Fig 1: Copper cage
containing folded Mg
ribbon
6) Tilt the gas collecting tube slightly, and fill it to
the top with tap water. Use the 150 mL beaker for
this. Pour water slowly so that it forms a layer over
the acid, avoid stirring up the acid layer in the
bottom of the test tube. The final liquid in the top
of the gas collecting tube will contain very little
acid. Fill the tube so full with water that the water
actually bulges upward from the lip of the gas
measuring tube. Use the water bottle for the last
few drops.
7) Place the copper wire cage in the open mouth of the
gas measuring tube as in figure 2a. The cage should
be positioned at about the 50 mL mark on the tube.
Hook the copper wire over the edge of the gas
collecting tube and clamp it there by inserting the
rubber stopper. When properly set up, the tube will
contain no air bubbles, and the water will
completely fill the hole in the stopper as well as the
tube.
8) Place your finger over the rubber stopper holes and
invert the gas measuring tube. DO NOT remove
your finger until you are under the water level in
the next step.
Figure 2
9) Place the inverted tube into the 600 ml beaker filled with water, then remove your finger. Clamp the
tube in place as in figure 2b.
10) The acid, being denser than water, will stream down through the water and eventually reach the Mg
ribbon, and react.
11) As you work, the teacher will read the barometer and thermometer. When the values are announced,
record the room temperature and pressure in your data table.
12) After the reaction ceases, let everything sit for 2-3 minutes before continuing. Try to dislodge any
bubbles clinging to the sides of the gas collecting tube and caught by the copper wire cage by tapping
the tube gently.
13) To measure the volume of gas produced, you
must ake the gas pressure inside the tube equal
to the room pressure BEFORE measuring the
volume. To do this, cover the hole in the stopper
with your finger and transfer the gas collecting
tube to to a 500 ml graduated cylinder. Raise or
lower the gas collecting tube until the water level
INSIDE the gas collecting tube is equal to the
water level in the graduated cylinder. Be careful:
do not raise the gas tube so high that its open
end comes above the battery jar's water level.
14) When the gas pressures are equalized, read the
volume of gas from the scale on the tube and
record this value in your data table. Please note
that zero is at the top of the tube. Read the
value from the top down, not from the 50 mL
mark going up.
15) Remove the gas collecting tube from the
graduated cylinder. Hold the gas collecting tube
over the 600ml beaker. Do not block the stopper
tube, firstly because there is now acid at the
bottom of the gas collecting test tube, and
secondly, because you are trying to empty the
contents of the gas collecting test tube into the
beaker. Add a few pieces of chalk to the solution Figure 3: Measuring the volume of gas. In b, room
in the beaker and wait until the reaction stops. pressure is equal to the combined pressure of
Then you may pour all liquids down the drain, and H2 (g) and H2O (g).
rinse all glassware three times with tap water.
When done rinsing, let the water run for 2 more
minutes.
16) Throw the wire cage in the trash. Do not keep it.
Data Table
magnesium ribbon length (±
mass of 1 meter of Mg (±
room temperature (±
Room pressure (±
cm)
g)
C)
kPa)
"wet" gas volume (±
mL)
Results
Sample calculation
mass of magnesium used
moles of magnesium used.
moles of hydrogen gas
produced
pressure of the "dry"
hydrogen gas
ж
Please see below for
some remarks on how to
calculate this
volume of "dry" hydrogen
gas at STP
molar volume (in
liters/mole at STP)
(according to the data
from your experiment)
percent error
result
Ж
Since hydrogen has collected over water,
the gas in the tube consists of a mixture of
hydrogen gas and water vapour. The total
pressure caused by these two gases is equal to
the room pressure. (see figure 4) Mathematically,
this can be expressed as:
PH2 + PH2O = Proom.
The pressure of the room can be determined by
reading a barometer. The pressure of the water
vapour
PH2O,
can be determined from table 1.
The values in table 1 were obtained by measuring
the pressure of water vapour above liquid water
at various temperatures.
Discussion: Answer these questions, then write
out your own discussion.
Q1: How did you know the “Mg + HCl” reaction
had begun?
Q2: After the “Mg + HCl” reaction ceased, why
did you let everything sit for 2-3 minutes before
continuing? To allow the tube and its contents to
come to room temperature, because T affects
the behaviour of gases.
Q3: You used Boyle’s law to calculate the volume
of hydrogen at STP, what 2 parameters must be
kept constant in order for Boyle’s law to apply?
Q4: Can a liquid have a molar volume? What
information would you have to know in order to
calculate it? Is the molar volume of a liquid
useful? Explain.
Conclusion: Write out your own conclusions.
Figure 4: Correcting for the vapour pressure of
water. Comparing the pressure in the gas collecting
of combined pressures of H2(g) and H2O(g) to the
pressure of H2(g) alone.
T
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Table 1
Vapour Pressures of Water
at Various Temperatures
kPa
T
kPa
0.61129
16
1.8185
0.65716
17
1.938
0.70605
18
2.0644
0.75813
19
2.1978
0.81359
20
2.3388
0.8726
21
2.4877
0.93537
22
2.6447
1.0021
23
2.8104
1.073
24
2.985
1.1482
25
3.169
1.2281
26
3.3629
1.3129
27
3.567
1.4027
28
3.7818
1.4979
29
4.0078
1.5988
30
4.2455
1.7056
For teacher version
Explanation of Discussion Answer Techniques
Some of the sample answers contain instructions for what to write in your work to be turned in. In
general, your work should be neat and showing all units. The values you write down MUST support the
answer you arrive at for each discussion problem.
The explanations for the discussion are based on the following sample data:
(a) Mass of 1.00 m of Mg ribbon: 0.809 g
(b) Length of ribbon used: 5.30 cm.
(c) room pressure: 752.3 mmHg
(d) room temperature: 23.0 °C
(e) volume of wet gas collected 42.5 mL
1) Mass of Mg used - you must use a proportion (two ratios set equal to each other). The data in (a) and
(b) are used. Using the sample data, the value arrived at is 0.0429 g.
2) Moles of Mg used - you must divide by the atomic weight of Mg and the answer is 0.00176 moles of
Mg.
3) Moles of H2 produced - this depends on the molar ratio between Mg and H 2. Show a balanced equation
in your answer to justify the ratio you arrive at. Then, indicate in writing how many moles of H 2 were
produced.
4) Corrected Pressure of H2 gas - this value is determined by using Dalton's Law of Partial Pressures:
Ptotal = PH2 + Pwater vapor
The total pressure is 752.3 mmHg and the Pwater vapor value must be looked up in a table. The value used
MUST be in mmHg.
The PH2 is the "corrected pressure" of H2 or the "pressure of the dry gas." It is the value that should be
used in the following calculation.
5) Volume of Collected Gas at STP - this calculation will use the Combined Gas Law where P1V1 / T1 = P2V2
/ T2.
The P2 and T2 values are the standard values, P1 is the corrected pressure (the one from #4), T1 is the
room temperature (remember: all temps in Kelvins!!) and V 1 is the volume of gas you measured.
Make sure you show how the problem is set up with all units included. The answer is 37.7 mL.
6) Molar volume of H2 - the molar volume of any gas at STP is known to be 22.414 L/mol. To calculate this
value, you should take your volume calculated in the step just above and convert it to liters. You do not
need to show this step. Then divide that value by the moles of H 2 that was determined in calculation #3. I
have deliberately left that value out, BUT I have told you how to get it.
I will not tell you the answer to #6, but when rounded to the nearest 0.1 place, it is within 1.0 of the
correct answer of 22.4.
7) Percent error - numerator: the absolute difference between the correct (usually called the true) value
and the value you measured (the experimental value). Denominator: the true value. Multiply by 100 to get a
percentage, round to the nearest 0.1 place. You're done!!
Q1: Effervescence
Q2: After the “Mg + HCl” reaction ceased, why did you let everything sit for 2-3 minutes before
continuing? To allow the tube and its contents to come to room temperature, because T affects the
behaviour of gases.