Measuring the Percentage
Concentration of Oxygen in Air
Object of the Experiment
We will try to make a measurement of the approximate percentage
concentration of
oxygen in air. Air is a mixture of several invisible gases, so we
first need to be able to separate just the oxygen from the other gases
in an air sample. As that is being done, there needs to be some
kind of a visible
change that you can see and measure.
Conducting the Experiment
Basically a moist piece of steel wool is stuck into
a glass 100 mL graduated cylinder. The cylinder is turned upside
down
and the open end is immersed in a cup of water. The air in the
graduated cylinder is sealed off from the rest of the atmosphere.
The oxygen reacts with the steel wool to form rust and is removed from
the air sample (it turns from a gas and becomes part of the rust, a
solid). As oxygen is removed from the sample, water will rise up
into the cylinder and its level can be read on the cylinder scale.
If you simply try to immerse the open end of the cylinder in a cup
of water you would find that the water doesn't enter the
cylinder. Air pressure keeps the water out. You want the
water to enter partway into the cylinder so that the water level can be
read on the cylinder scale.
Note that it isn't that the cylinder is full of air
that
keeps the
water out (as shown above at left), there's actually a lot of empty
space in the cylinder. Rather it is the fact that the air
molecules are moving around inside the cylinder at 100s of miles per
hour and they strike the water molecules with enough force that the
water can't move into the cylinder (the more accurate drawing above at
right). The pressure exerted by the
air in the cylinder is what keeps water from rising up into the
cylinder.
The solution to this problem is to insert a small piece of
flexible tubing into the cylinder as shown above. If you lower
the cylinder into the water while keeping the two ends of the tubing
out of the water, water will enter the cylinder. When the water
level can be read on the scale (ideally between the 90 and 100 ml
marks), the tubing is removed. This seals off the air sample and
the experiment is underway.
You can carefully rest the cylinder against bottom and side of the
cup. Leave the experiment materials in a location where they
won't be disturbed; it may takes several days for all of the oxygen to
be completely removed from the air sample.
Periodically lift the cylinder just enough to be able to read the
water level. Don't lift the open end of the cylinder out of the
water as this would break the seal and you would need to restart the
experiment. Also make a note of the time.
After some time you will notice that the water level doesn't
change between readings. All of the oxygen in the sample has been
removed and the experiment is over. The figure below shows you
one way of removing the steel wool (which should then be
discarded).
Straighten a paper clip and
then bend
about 2/3 rds of it around the end of a pencil to form a
corkscrew. Attach the corkscrew to the end of the pencil and then
insert it into the cylinder. With a list twisting the corkscrew will
snag the steel wool and you will be able to pull it out of the cylinder
and dispose of it.
Data Analysis
Imagine you could measure Ninitial,
the
total
number
of
air
molecules
in
the air sample at the start of the experiment. During the
course of the experiment oxygen in the air reacts with the steel wool
to form rust; oxygen gas is removed from the air sample. There is
a decrease in the number of air molecules in the sample.
You could determine Noxygen, the
number of oxygen molecules, by
subtracting Nfinal from Ninitial:
Nfinal
is the number of
air
molecules remaining at the end of the experiment after the oxygen has
been removed.
The oxygen concentration could then be found using the following
equation:
The problem is that you can't directly measure Ninitial
or Nfinal(the air molecules are invisible and there are way too many to
be
counted anyway). This experiment is set up in such a way that the
pressure of the air in the air sample remains constant even though the
oxygen is being removed from the sample (water moves into the graduated
cylinder as the O2 is removed
which decreases the volume of
the air
sample). Because pressure and temperature remain constant the
ideal gas law
can be written in the following way:
The air sample volume is directly proportional to N (the term in
parentheses remains constant). A change in N causes the same
percentage change in V. The volume of the air sample is something
you can see and measure. You can determine the oxygen
concentration using measurements of volume:
A graph of some sample data and a
sample
calculation are shown below
