Experimenting With Gases
Introduction:
We already know how to work gas problems using the gas laws. Now we
need to consider the actual experimental collection of gases so we can do some
experiments ourselves. How can we collect a gas and work with it?! We can pour
a liquid into a beaker and mass it or pour the liquid into a graduated cylinder and
read the volume. We can easily place a solid on a balance. However, we can't
pull a gas out of the air and hold it on a balance nor can we pour a gas into a
graduated cylinder. Gas molecules are small, invisible, and constantly moving.
What do we do?!!! In basic experimental work, gases are "collected over water."
Many gases are not soluble in water and can be collected by bubbling them
through water and into a gas collection tube. This will allow us to trap, "see", and
measure the gas molecules.
Experimental Strategy:
Suppose we are collecting hydrogen gas. Before the reaction starts, the
gas collection flask is filled with water. When the gas starts to flow from the
delivery tube into the collection tube, hydrogen bubbles rise to the top of the tube
which forces some of the water out. This is called "water displacement." After the
reaction is over, we need to know how much gas we have collected. To figure
this out, we will need to know the temperature, volume, and pressure.
1. Temperature - measure water temperature.
2. Volume - read from the scale on the side of the collection tube which is
graduated in mL.
3. Pressure - there are two possible scenarios.
Scenario I: Equal Water Levels
The first scenario is that the water left inside the collection tube is equal to
the level of the water outside the tube. When these two levels are equal, the
pressure (or push) of the gas on the inside must equal the pressure (or push)
from the atmosphere on the outside. Neither gas is winning in their ability to push
the water - it is a standoff. If we measure the pressure of the atmosphere by
reading a barometer then we will also know the pressure of the collected gas,
right? Well, sort of.
In the equal level scenario, Patm will reflect the pressure of the gas inside
the tube, but the problem is that there is more than one gas inside the tube.
When a gas is bubbled through water, the collected gas always is "contaminated"
with water molecules that have evaporated into the gas phase. Water vapor, like
other gases, exerts a pressure. We call it the vapor pressure of water and we
have to take this into account.
Since the gas pressure is due to the collision of the various gas molecules
with the walls of the container, the total pressure of the mixture of gases (the
collected gas and the water vapor) is the sum of their partial pressures. This
follows from Dalton's Law of Partial Pressures. The partial pressure is the
pressure each gas would exert if it alone were present. To determine the
pressure of the "dry" gas (Pgas  unmixed with water vapor), the vapor pressure
Chemistry II
Cary Academy
W.G. Rushin
1
of water (PH2O) at the given temperature is subtracted from the total pressure
(Ptotal) of the gas within the tube (Pgas = Patm - PH2O).
Temp
(*C)
0
5
10
11
12
13
14
15
16
Vapor Pressure of Water
Pressure Temp Pressure Temp
(torr)
(*C)
(torr)
(*C)
4.6
17
14.5
30
6.5
18
15.5
40
9.2
19
16.5
50
9.8
20
17.5
60
10.5
21
18.7
70
11.2
22
19.8
80
12.0
23
21.1
90
12.8
24
22.4
100
13.6
25
23.8
Pressure
(torr)
31.8
55.3
92.5
149.4
233.7
355.1
525.8
760.0
1. A student generates oxygen gas in the lab and collects it over water at 25.8*C
until the levels of the water inside and outside the collection tube are equal. If
the volume of the gas is 245.3 mL and the barometric pressure is 758.6 torr:
a. What is the partial pressure of oxygen gas in the “wet” gas mixture?
b. What would be the volume of dry oxygen at STP?
Scenario II: Unequal Water Levels
In this scenario, the levels of water inside and outside the collecting tube
are unequal. This adds an additional correction factor because the total pressure
of the collected gas mixture is no longer equal to P atm. We can correct for this by
measuring the difference in the levels with a ruler in mm H2O. However,
barometric pressure is measured in mm Hg, not mm H2O. Since Hg is 13.6 times
as dense as water, a given pressure will support a column of water 13.6 times as
high as an equivalent column of Hg. Therefore, a difference in water levels must
be divided by 13.6 to give its equivalent height in mercury.
Pdiff = mm H2O = mm Hg
13.6
Depending on whether the H2O level is higher inside the collection tube or
outside the collection tube, the calculated Pdiff will have to be subtracted or added
from Patm. We will still have to correct for PH2O in addition to Pdiff.
Pgas = Patm - PH2O - Pdiff
or
Pgas = Patm - PH2O + Pdiff
2. 50.0 mL of oxygen are collected over water. The level of water is 27.2 mm
higher inside the collection tube than outside. The temperature is 20.9 *C,
and the barometric pressure is 754.3 torr. What is the volume of the dry gas
at STP?
Chemistry II
Cary Academy
W.G. Rushin
2