Measuring Pressure
The gas laws relate the physical properties of volume, pressure, temperature, and amount of
substance to each other.
Pressure
There are two main types of pressure: ____________________ and the pressure of a gas
inside a container.
1) Atmospheric pressure is the pressure caused by the _____________________________
and is measured with a barometer.
How a barometer works
An evacuated tube is filled with _______________________ and then the open end is
immersed in a pool of mercury. __________________ pulls down on the liquid level inside
the tube. At the same time the _________________________ pushing down on the
mercury pool tends to force the mercury _______________________________. The two
forces balance out and the height of the mercury column stabilizes. This height of the
mercury above the surface of the pool is called the ______________________________.
At sea level it is ___________________ high.
760 mm Hg = 1 atm
1 mm Hg = 1 torr
760 torr = 1 atm
1 atm = 101.325 kPa (SI)
1 atm = 14.69 psi
Notice that _______________________ is used in barometers rather than other liquids
because it is so ___________________. This means that the mercury will not rise too
high. At the same temperature, the ratio of _________________ of the liquid columns is
_____________________ proportional to the ratio of the liquids’ __________________.
Example:
The barometer reading in Mrs. Darling’s classroom on September 4, 2008
was 104.1 kPa.
a) Convert this measurement into mm Hg.
b) If Mrs. Darling’s barometer used ethanol (d = 0.80 g/mL) instead of
mercury (d = 13.6 g/mL), how high would the column of ethanol be?
2) The pressure of a gas inside a container is measured with a ________________________.
How a manometer works
As with the barometer, the pressure of a gas is balanced against a column of ___________.
There are two types of manometers: _______________________________ and
________________________________. In open tube manometers you are essentially
comparing the pressure of the gas inside the container to _________________________
____________________________. In the diagram below, the first manometer shows a
gas with a pressure ____________________ atmospheric and the second manometer
shows a gas with a pressure ______________________ atmospheric.
Example:
Calculate the pressure of a gas if barometric pressure is 748.2 torr and the
mercury in the arm open to the atmosphere is 7.8 mm higher than the other
arm.
In closed tube manometers you are essentially comparing the pressure of the gas inside the
container to a _______________________. This type of manometer is generally used for
measuring low gas pressures and there is no need to correct for atmospheric pressure.
Example: A scientists heats a solid sample of CaCO3 and collects the CO2 formed in an
evacuated, closed- end manometer. At room temperature the change in height of
mercury is 313.2 mm Hg. Calculate the pressure of the CO2 in atmospheres.