The Kinetic Molecular Model provides a simple, qualitative method
for estimating what will happen to a gas when it is heated, cooled,
compressed or allowed to expand. However, if we want to calculate
the exact results of these changes, we must combine
Boyle's, Charles', and Avogadro's individual gas laws together into one
unified law of ideal gas behavior:
In this equation, P is the
pressure in atmospheres, V is the volume
in liters, n
is the number of mols of gas, T is the
absolute temperature in Kelvins, and R is the ideal
gas constant (0.082 L•atm/mol•K).
The ideal gas law is based on the assumption
the molecules experience no intermolecular forces and that the
themselves occupy no volume. These assumptions are valid at low
and high temperature. Under these conditions, the molecules are too far
apart to "feel" attractive forces exerted by other molecules.
since the molecules are far apart, the volume occupied by the molecules
themselves is negligible compared to the total volume occupied by the
In reality, intermolecular forces do
exist and molecules do occupy
The extent to which these factors cause a gas to deviate from the ideal
gas law at a particular temperature and pressure will depend upon its
structure. These deviations become severe when a gas is subjected
to high pressure and/or low temperatures. Under these conditions,
a gas begins to take on the characteristics of a liquid and the ideal
law no longer applies.
The purpose of this experiment is two-fold: first, to test
your understanding of the Ideal Gas Law and second, to ascertain the
level of laboratory skills you have developed over the semester.
To accomplish this, you will be generating carbon dioxide gas under
controlled conditions. With the proper technique, and a little
algebra, you should be able to parley your observations and data into
four very important results:
- The molecular weight of carbon dioxide (CO2).
- The Ideal Gas Law constant 'R'.
- The molar volume of carbon dioxide (CO2) at STP.
- The amount of sodium bicarbonate (NaHCO3)
an Alka-Seltzer tablet.
At first glance it would appear that these are four very different
experiments. However, all four values can easily be
determined from the reaction of an Alka-Seltzer tablet with
hydrochloric acid. One of the active ingredients is sodium
bicarbonate which readily reacts with hydrochloric acid to produce
HCl NaCl + CO2
(of course, you remember this reaction from the Mole Concept lab
earlier in the semester...Don't you?!?).
So by simply measuring the amount of carbon dioxide generated
from a given tablet and knowing the temperature and atmospheric
pressure in the room, all of these results can be determined. Now
that is what I call...Fun with the Ideal Gas Law!!
Although you are trying to achieve four goals, the experiment
itself is elegant in its simplicity. It is loosely based on the
method for determining the
molecular weight of low boiling liquids (Section 12.10 of your
textbook). However, instead of a liquid, you will be determining
the molecular weight (molar mass) of a gas.
Careful inspection of an Alka-Seltzer package reveals that each
tablet contains three active ingredients: acetylsalicyclic acid
(aspirin - remember synthesizing that earlier this semester?), citric
acid (a weak, triprotic acid), and sodium bicarbonate. When the
tablet is dissolved in water, some of the sodium bicarbonate reacts
with the acetylsalicyclic acid to produce sodium acetylsalicylate
(which is more water soluble than aspirin and so is absorbed by the
blood stream more quickly). The sodium
bicarbonate also reacts with the citric acid to produce sodium citrate
and the trade mark bubbles of carbon dioxide. Even after both of
these reactions are complete, there is still excess sodium bicarbonate
remaining which as a weak base can react with excess stomach acid
(hydrochloric acid) to relieve heart burn, etc.
To achieve all the goals of this experiment, you will first grind an
Alka-Seltzer tablet to fine powder. A
portion of this power will then be placed into an empty gelatin
The capsule is then placed in a test tube containing
hydrochloric acid. The test tube/HCl/capsule is then placed in a
sealed plastic wash bottle containing distilled water. It will
take a couple of minutes for the HCl to dissolve the capsule.
Once it does the
hydrochloric acid will react with the sodium bicarbonate. The
dioxide produced will build up pressure in the bottle which will force
water out of the bottle. The amount of carbon dioxide produced
can then be calculated from the volume of water displaced:
It is important to use hydrochloric acid because it is a strong acid
and will completely react with all of the sodium bicarbonate before it
can react with the aspirin or the citric acid (a weak acid).
OK, the first task is to determine the molecular weight of carbon
dioxide (yeah, yeah, I know its 44g/mol, but what does YOUR data
say?). Lets start with the Ideal Gas Law equation and solve for
Remember that the number of mols
of any substance can also be
calculated by dividing the weight (in grams) of a compound by its
Since both expressions are equal to n,
they must be equal to each
This can be rearranged to give us the
molecular weight of the carbon dioxide
So to calculate the molecular weight of carbon dioxide, we need to
know the weight of carbon dioxide present, its absolute
temperature, its partial pressure,
and its volume. You will be measuring all of these variables
during the lab. For the calculation of carbon dioxide's MW, we
will assume that R is a constant
All right, one down, three to go. To calculate the MW of carbon dioxide above, we
assumed that the Ideal Gas Law constant, R, had a value
of 0.082 L•atm/mol•K. Now we will assume that the MW of carbon
dioxide is 44g/mol, and use this to calculate R. Again, starting with
the Ideal Gas Law equation, we can simply isolate R:
So calculate the value of R, we need to
know the partial pressure of carbon dioxide, its absolute temperature,
and its volume. Again, you know all of these. For this calculation, we will assume that the molecular
weight of carbon dioxide is 44g/mol, which will allow us to calculate
the mols needed for this calculation. The only major hurtle is
that 'partial pressure' thing. The carbon dioxide gas produced in
displaces water into the beaker. After the gas generation is complete,
pressure inside the bottle is equal to the atmospheric
pressure. At this point, the total pressure inside the
bottle, according to Dalton's Law of Partial Pressures, is equal to the
sum of the partial pressures of carbon dioxide (PCO2) and
water vapor (PH2O):
You can easily read the atmospheric, Patm, from the
barometer in the lab (How
to read an Eco-Celli
barometer), but we need to know PCO2 to calculate R. It
turns out that there are tables that document the vapor pressure of
water at various temperatures (Vapor
Pressure Table for Water). So, simply subtract the
partial pressure of water vapor from the atmospheric pressure measured
by the barometer and you have the partial pressure of carbon
dioxide. Use that as the pressure, and you can calculate R. Q.E.D.
Two down, two to go. Now we would like to know the molar volume, V*, of carbon
dioxide at STP (Standard Temperature and Pressure):
Of course you remember that the conditions at STP are zero degree
Celsius and one atmosphere of pressure. Since you did not run
this experiment at those conditions, you will have to use the combined
gas law to figure out what the volume would have been:
If carbon dioxide is indeed an ideal gas (and you did the lab
correctly), the molar volume at STP should be 22.414 l/mol.
Three down, one to go! Now we want to know what percent of that
Alka-Seltzer tablet was sodium bicarbonate. Since we know how
much carbon dioxide was produced, we can convert that to grams of
sodium bicarbonate using the balanced chemical equation for this
reaction (Grams to Mols to Mols to Grams....sound familiar). Now
simply divide the grams of sodium bicarbonate, by the total mass of the
Alka-Seltzer used in each reaction and you will have the percent
What did I say....Kinder spiel!
Please read the following procedure
carefully before coming to
This experiment is technique intensive. None of the individual
are difficult, however, if they are not performed in the proper order,
spurious results will be obtained. As always, measure all
temperatures to 0.1°C and all masses to 0.001g.
- Carefully clean and dry a 150 mL beaker, a 250 mL beaker, and a
large 155mm test tube.
- Remove two Alka-Seltzer tablet from their hermetically sealed
packet and weight them.
- Use a clean (use paper towels only, NO water) mortar and pestle
to grind the tablets into a fine power.
- Accurately weigh an empty gelatin capsule. Note: make
sure your hands are dry and that you close the bottle containing the
capsules IMMEDIATELY! Gelatin starts dissolving when it comes in
contact with moisture.
- Take the capsule apart and use your scoopula to fill the larger
portion with your powdered Alka-Seltzer.
- Put the two parts of the capsule back together, carefully blow
off any power than may be sticking to the outside and reweigh.
- Add a magnetic stir bar to your test tube and place it upright
in your 150 mL beaker. Use a repipetter to add 10.0 mL of 6M HCl to the test test.
- Weigh the beaker, test tube, stir bar and HCl.
- Fill your water bottle to about half an inch below the 'full line' with distilled water
and place it on the magnetic stirrer motor.
- Weigh your clean, dry 250 mL beaker.
- Place it on a your iron ring with the wire gauze, and
position it so it can catch
all of the water from the water bottle.
- Remove the top from your water bottle and place the test
tube/stir bar/HCl in the bottle, it should float.
drop the Alka-Seltzer filled gelatin capsule into the
test tube and seal the water bottle. Be sure that the top is on
tight or some of the carbon dioxide will escape. Make sure the
spout of the
water bottle is pointed into the 250 mL beaker.
- Now adjust the magnetic stirrer motor to agitate the capsule and
HCl. It may take a minute or two for the capsule to dissolve and
the reaction to begin.
- As the reaction proceeds you should see a 'foam' produced.
At the same time, water will be
displaced from the water bottle into the 250 mL beaker.
- Keep adjusting the rate of the magnetic stirrer up and down for about 10
minutes or until all of the 'foam' has disappeared and no more water is
- After the reaction is complete, weigh the 250 mL beaker. By
difference, you now have the mass of water generated.
- Open the water bottle and carefully remove the test
tube/stirrer/HCl. Use a paper towel to wipe off any water from
the outside of the test tube.
- Place the test tube upright in the same 150 mL beaker and
reweigh. By difference, you now have the mass of CO2
- Record the temperature of the water in the bottle (to
You can use a C.R.C. or this Table
of Water Density to find the density of water at this
temperature. You will use this to convert the grams of water you
obtained in Step #19 into mL. This is the volume of CO2
- Record the barometric pressure in the lab (How to read an Eco-Celli barometer).
- Clean the test tube thoroughly to remove any remnants of the
gelatin capsule. Rinse with distilled water and thoroughly dry
with paper towels.
- Repeat Step #4-22 until you have at least three GOOD
trials. You can perform a simple calculation to determine if a
given trial is 'good' or not. Divide the amount of water generated by the mass of CO2 generated. This value should be in the range of 500-600 and should not vary by more than 10-20 units from trial to
trial. If it does, you have made some mistakes and will have to run additional trial(s).
For EACH trial, calculate the following
from your data (show all
- The mass, volume, and mols of carbon dioxide generated.
- The molecular weight of carbon dioxide.
- The ideal gas law constant, R.
- The molar volume of carbon dioxide at STP.
- The percentage of sodium bicarbonate in Alka-Seltzer.
- How did your trials compare to each other?
- How did your results compare to the accepted values? Calculate the percent error.
- What do you think is the biggest source of error in this
(Updated 9/13/13 by C.R.