Exp. 1A GASES: Avogadro’s Law
CHEMISTRY 206
Experiment 1: GASES
Instructor’s Informal Preamble
In our first laboratory session in Chem 206, we will be performing experiments that probe the
physical behaviour of gases. This topic was covered in Chem 205, so be sure to review the
appropriate chapter in your textbook.
It is important to remember that the molecules in any substance are constantly moving, and that
the average kinetic energy of the molecules is proportional to the temperature. The most
noteworthy feature of substances in the gas phase is that the molecules are so far apart that they
do not interact with each other significantly. The result is that all substances behave more-or-less
the same way in the gas phase, regardless of the chemical structures of their constituent
molecules. In the 17th and 18th centuries, the scientists Avogadro, Boyle and Charles made
general observations of gases, which they summarized as the natural laws of gas behaviour. You
will likely recall the form of these natural laws that we use most often: the ideal gas law (PV =
nRT), which shows how the quantity (in moles, n) and temperature (T) of a gas determine the
volume (V) it occupies and the pressure (P) it exerts.
Experiments involving gases are a useful starting point for the laboratory work in Chem 206
because of their quantitative nature. This week’s experiments give you a good opportunity to
focus on mastering your observation and measurement skills and practicing making careful and
well thought-out calculations using simple, familiar concepts. The rest of the experiments in
Chem206 depend on you being able to make careful quantitative measurements and calculations
involving more sophisticated concepts – so it is very important that you make a considerable
effort to become comfortable with quantitative work this week.
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Exp. 1A GASES: Avogadro’s Law
CHEMISTRY 206
Experiment 1A: AVOGADRO'S LAW
Introduction
Amedeo Avogadro (1776-1856) was an Italian scientist who helped answer some of the basic
questions about why substances react in only certain proportions. In particular, he helped solve
the mystery of why gases react with each other in small whole-number volume ratios. He
proposed that equal volumes of any two gases contain equal numbers of molecules, no matter
what the substances are, as long as they are at the same temperature and pressure.
A more formal general statement of Avogadro's hypothesis is:
At constant temperature and pressure, the volume (VP,T) occupied by a gas
is proportional to the number of molecules or moles of molecules (n) that it contains.
Expressed mathematically:
or,
VP,T ∝ n
where subscripts denote constant parameters
and ∝ means "is proportional to…"
VP,T = an
where a is a proportionality constant
This mathematical expression is often referred to as Avogadro's Law. This is an example of a
natural law, a "rule" that describes typical behaviour and is derived from experimental
observations. In this experiment, you will test the validity of Avogadro’s law.
Objective of the Experiment
Your goal is to answer the question, “How well is the behaviour of a real gas described by this
natural law?” To do this, you will generate several known quantities of carbon dioxide by
reacting dilute acetic acid solution with sodium hydrogen carbonate (sodium bicarbonate,
"baking soda") and then measure the volume occupied by the gas.
Prelaboratory Assignment
Carefully read the procedure and answer the prelaboratory questions before coming to the lab.
The questions include the calculation of the maximum quantity of sodium hydrogen carbonate to
be used in the experiments. It is essential that you verify that your answer is correct before
proceeding. Your demonstrator will inspect and collect your prelab before you are permitted to
begin the experiment - keep a copy of it for yourself, and have the TA sign your receipt record.
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Exp. 1A GASES: Avogadro’s Law
Experiment Summary
In a bottle connected to a syringe, a measured quantity of sodium hydrogen carbonate will be
mixed with an excess of acetic acid. The reaction occurs according to the following equation:
NaHCO3(s) + CH3COOH(aq) → NaCH3COO(aq) + H2O(l) + CO2(g)
This reaction can be summarized more briefly by using the net ionic equation:
HCO3-(aq) + H+(aq) → H2O(l) + CO2(g)
The carbon dioxide produced will push up the syringe piston, and its volume can thereby be
measured. Knowing the mass of sodium hydrogen carbonate used, and thus the number of
moles, you can calculate the mass and hence the number of moles of carbon dioxide that will be
produced. Five runs are done using different quantities of sodium hydrogen carbonate so that an
average value for the molar volume of carbon dioxide (or any other gas!) can be calculated.
Materials
Apparatus
•
•
•
bottle and syringe setup (see diagram to the right)
o 250 mL plastic bottle
o One-hole stopper fitted with a short glass tube
o Short length of rubber tubing
o 60 mL syringe
semi-micro (4") test tube
25 mL measuring cylinder
Reagents and Disposables
•
•
•
sodium hydrogen carbonate
1 M acetic acid solution
silicone oil (to lubricate the syringe)
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Exp. 1A GASES: Avogadro’s Law
Procedure
1. Obtain and inspect your syringe – if it is sticky with old lubricant, you must wash it well
with soap and water before proceeding. Next, lubricate the rubber piston of the syringe
with a little silicone oil so that it slides smoothly (very important!) in its barrel. Connect
the syringe to the glass tube in the stopper with a short length of rubber tubing. Clamp
the syringe loosely on stand. Pull about 50 mL of air into the syringe and then fit the
stopper into the neck of the bottle. To make sure the system is air-tight, push in the
syringe piston to about the 10 mL mark, wait a few seconds and then release it. It should
go back to roughly its original position. If not, either air has leaked out of the system, or
the piston is not moving freely. Try "helping" it gently to go back to its original position
to test the freedom of movement; you might need a little more silicone oil. If the problem
seems to be a leak, check the connections.
The following steps [(2) - (5)] will be repeated with the five quantities of sodium carbonate
calculated in pre-lab question 2.
2. Tare the bottle and then add the appropriate mass of sodium hydrogen carbonate required
for the run. The mass taken need not be exactly what you calculated - to within ±20% is
good enough, but record the mass accurately to the nearest milligram.
3. Measure out 8 mL of 1 M acetic acid solution with a graduated cylinder and pour it into
the test tube. Make sure there is no solution on the outside (i.e., dry tube if necessary),
and carefully place the tube upright in the bottle, without spilling the contents.
4. Move the syringe piston close to the zero mark. Reconnect it to the bottle and read the
scale at the bottom of the piston. (It should still be near zero.) Then, by tilting the bottle,
cause the acetic acid solution to spill and react with the sodium hydrogen carbonate.
Carbon dioxide will be evolved, and the syringe piston will be driven out. [If the piston
comes all the way out, you have really messed up your pre-lab calculations!] As the
reaction slows, agitate the bottle a few times to make sure all the sodium hydrogen
carbonate has reacted. Gently move the syringe piston to make sure it is sliding freely so
that it reaches a position where the pressure is the same inside and outside the apparatus.
[This is critical to the success of the experiment.]
5. When the reaction is complete, record the new reading on the syringe, and report the
volume of carbon dioxide that has been evolved. Disconnect the bottle, then rinse the
bottle, the vial, and the test tube to be ready for the next run. The spent reaction mixture
can be washed down the sink. Dry the bottle carefully on the outside before the next
weighing. Repeat this procedure starting at step (2) for the other starting quantities of
sodium carbonate.
When this experiment is complete, proceed with experiments 1B & 1C. Remember to clean all
glassware, other apparatus, and your work space when finished.
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Exp. 1A GASES: Avogadro’s Law
Name: _____________________________ Section: ______________ Date: ________________
CHEMISTRY 206
Experiment 1A: AVOGADRO'S LAW
Prelaboratory Questions
1. (1 mark) Provide a half-page (maximum) summary of the experimental procedure. This
can take the form of a flow-chart or a list of the main steps in the procedure. DO NOT
simply copy the procedure directly from the manual.
2.
(1 mark) The maximum volume of gas that could be measured by the syringe used in this
experiment is 60 mL, if the syringe starts empty. Calculate the mass of sodium hydrogen
carbonate, NaHCO3, required to generate 60 mL of carbon dioxide, CO2, at 1 atm
pressure and 25 oC. To do this calculation you will need to know that 1 mole of an
(ideal) gas occupies 22.4 L (or, just use the gas constant, R, and the ideal gas law!). Note
that the balanced chemical equation for the reaction was provided earlier and that this is
just a simple stoichiometry problem. [NOTE: In the five reaction runs you do for your
experiment, you should use approximately 1/5, 2/5, 3/5, 4/5, and 5/5 of the quantity you
calculate here.] Important: for this and subsequent calculation-style questions in this
course, please show detailed calculations, including units as appropriate.
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Exp. 1A GASES: Avogadro’s Law
3. (1 mark) The CRC handbook of Chemistry and Physics gives the solubility of carbon
dioxide in water at 25 oC as 0.145 g in 100 mL. What is this concentration in moles⋅L-1?
4. (1 mark) In the experiment, you will use 8 mL of acetic acid solution to react with the
sodium hydrogen carbonate. Assuming that the solubility of carbon dioxide is the same
in acetic acid solution as in water, how much carbon dioxide could be dissolved at 25 oC?
5. (1 mark) What volume, at 25 oC and 1 atm, would be occupied by the mass of carbon
dioxide calculated in question 4 if it were not dissolved in solution?
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Exp. 1A GASES: Avogadro’s Law
Name: _____________________________ Section: ______________ Date: ________________
CHEMISTRY 206
Experiment 1A: AVOGADRO'S LAW
Laboratory Report
(5 marks) Observations and Data
Mass of
Run
NaHCO3 (g)
Volume of
CO2
measured
(mL)
Mass of
Run
NaHCO3 (g)
1
4
2
5
3
Temperature (oC)
Volume of
CO2
measured
(mL)
Observations - This space is to report any particular difficulties you had.
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Exp. 1A GASES: Avogadro’s Law
Conclusions and Interpretation of Results
(3 marks) Calculations
For each run, calculate the number of moles of carbon dioxide that should have been generated
based on the sodium bicarbonate used. Next, calculate the volume occupied per mole of CO2
(molar volume) in each of your runs. Calculate the average molar volume, the deviation of the
individual runs from the average, and the average deviation. [Recall: deviation is the difference
between the observed quantity and its average value; average deviation is the average of these
deviations].
Run
Theoretical amount of CO2
(moles)
Molar Volume Deviation
Volume of CO2
(from table above) (mL)
(L⋅mol-1)
1
2
3
4
5
Average:
(3 marks) Provide a detailed sample calculation for one of the runs:
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Exp. 1A GASES: Avogadro’s Law
Conclusions and Additional Questions
1. (1 mark) Do your individual runs support the correctness of Avogadro's Law? Explain.
2. (1 mark) How does your average value of the molar volume differ from the accepted
value you were given in the prelab? Does it differ by more than the average deviation in
your experimental runs? Comment on what this tells you about the degree of error in
your experimental results.
3. (2 marks) Is the amount of carbon dioxide that would have been dissolved in the acetic
acid solution enough to explain the difference you observe? What other aspects of the
experimental design might contribute significantly to differences between the accepted
value and your experimental value?
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