Graham’s law
states that the rate of diffusion or
effusion of a gas is inversely proportional to the
square root of its molar mass.
1
M
Rate
Gas Pressure
Pressure is defined as the force applied per unit area:
force
pressure =
area
The SI unit of force is the newton (N), where
1 N = 1 kg•m/s
2
The SI unit of pressure is the pascal (Pa), defined as 1 newton per
square meter.
1 Pa = 1 N/m
2
Definition and Units of Pressure

CHM2045
Gases
4
Calculation of Pressure
A
barometer
is an instrument that is used to measure atmospheric pressure.
Standard atmospheric pressure
(1 atm) was originally
defined as the pressure that would support a column of
mercury exactly 760 mm high.
P = hdg
h =
height in m
d
= is the density in kg/m
3
g
= is the gravitational constant (9.80665 m/s
2
)
Measurement of Pressure
A
barometer
is an instrument that is used to measure atmospheric pressure.
1 atm* = 101,325 Pa
= 760 mmHg*
= 760 torr*
= 1.01325 bar
= 14.7 psi
* Represents an exact number
A
manometer
is a device used to measure pressures other than atmospheric pressure.
4.
Calculate the pressure exerted by a column of mercury 70.0 cm high. Express the pressure in
pascals, in atmospheres, and in bars. The density of mercury is 13.5951 g/cm
3
.
5.What pressure (in atm) is exerted by a column of mercury exactly 1 m high?
6.What would be the height of a column of water supported by the pressure you calculated in the problem 4? Assume that the density of the water is 1.00 g/cm3.

7.
If a skin diver takes a breath at the surface, filling his lungs with 5.82 L of air, what volume will
the air in his lungs occupy when he dives to a depth where the pressure of 1.92 atm? (Assume
constant temperature and that the pressure at the surface is exactly 1 atm.)
8.
Calculate the volume of a sample of gas at 5.75 atm if it occupies 5.14 L at 2.49 atm. (Assume
constant temperature.)
9.At what pressure would a sample of gas occupy 7.86 L if it occupies 3.44 L at 4.11 atm? (Assume constant temperature.)

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- Summer '14
- Chemistry