Vapor Pressure for gasses
collected over water
(an application of Dalton’s Law)
Vapor Pressure
Water has a vapor pressure. This is
caused by the evaporation of water
into a gas. This water vapor (gas)
above a container of water contributes
to the air pressure in the container.
Vapor Pressure
Often gasses are collected in a container
above water because the gas can
displace the water easily, but cannot
escape.
Vapor Pressure
One can read the volume of water
displaced, but some of the gas that has
been collected is actually water vapor
and must be subtracted from the
volume of the gas of interest.
Vapor Pressure
Water has a substantially lower vapor
pressure at lower temperatures, so one
needs to take temperature into
account. We do this with a vapor
pressure chart.
Vapor Pressure
For example:
If a gas is collected over water at a
temperature of 23C and a pressure of
642 mm Hg, what is the pressure of
the gas?
Vapor Pressure
At 23C the vapor pressure of water is 21.1 mm
Hg.
Subtract from the pressure:
642 mmHg – 21.1 mmHg = 620.9 mmHg
Vapor Pressure
A sample of gas weighing 5.61 grams
was collected over water at 42C at a
pressure of 170.4 kPa, and was found
to have a volume of 688 ml. Calculate
the molecular weight of the gas. The
gas was found to be an element.
Identify it.
Vapor Pressure
We note that the gas was collected over
water, so we need to correct for the
vapor pressure of water. However,
our table is in mmHg and the data is in
kPa, so we need to convert.
Vapor Pressure
170.4 kPa 760 mmHg
= 1278 mmHg
101.325kPa
Vapor Pressure
Now we can subtract the vapor pressure
of water at 42 C.
At 42C, the vapor pressure of water is
61.5 mmHg
Subtracting gives:
1278 – 61.5 = 1217 mmHg
Vapor Pressure
Now we look at what we know:
There is information about pressure,
volume, temperature and grams. It
wants to know molar mass, which
involves moles and grams.
Vapor Pressure
So we need PV = nRT
Vapor Pressure
PV = nRT
P = 1216 mmHg
V = 688 ml = .688 L
n=x
R = 62.36
T = 315 K
Vapor Pressure
Plug in and solve
(1217)(.688) = x (62.36)(315)
837 = 19643x
x = .0426 moles
Vapor Pressure
Molar mass is grams/moles
5.61 /.0426 = 131.66g/mole
If this is an element, it should be Xe