Gas Variables
Physical Characteristics
Typical Units
Volume, V
liters (L)
Pressure, P
atmosphere
(1 atm=101.3 kPa=760 Torr)
Temperature, T
Kelvin (K)
Number of atoms or
molecules, n
mole (1 mol = 6.022x1023
atoms or molecules)
Robert Boyle
Showed that Pressure
influences the volume
of a gas during a
controlled experiment.
Robert Boyle
Ireland
January 25, 1627 – December 30, 1690
Boyle's experiment
Torricelli's experiment did more than
just show that air has weight; it also
provided a way of creating a vacuum
because the space above the column
of mercury at the top of the
barometer is almost completely
empty. (It is free of air or other gases
except a negligible amount of
mercury vapor.) Torricelli's work with
a vacuum soon caught the eye of the
British scientist Robert Boyle.
Boyle's Experiment
Trapped air
Boyle's most famous experiments
with gases dealt with what he
called the "spring of air." These
experiments were based on the
observation that gases are elastic.
(They return to their original size
and shape after being stretched or
squeezed.) Boyle studied the
elasticity of gases in a J-tube
similar to the apparatus shown in
the figure below. By adding
mercury to the open end of the
tube, he trapped a small volume of
air in the sealed end.
Boyle’s Law: P1V1 = P2V2
Boyle’s Law: P1V1 = P2V2
Boyle’s Law: P1V1 = P2V2
A Boyle’s law graph will always have a ½
hyperbola shape. Illustrated by
the equation  PV
=k
Volume and pressure are inversely proportional.
If one increases the other decreases.
Boyle's Law
The pressure of a gas is inversely
related to the volume when the
Temperature does not change
Then the PV product remains constant
P1V1
=
P2V2
A Boyle’s Law Problem
Freon-12, (CCl2F2), is used in refrigeration systems.
What is the new volume (L) of a 1.6 L sample of
Freon gas initially at 50 mm Hg after its pressure is
changed to 200 mm Hg at constant T?
Initial conditions
Final conditions
P1
= 50 mm Hg
P2
= 200 mm Hg
V1
= 1.6 L
V2
= ?
V2 = 1.6 L x 50 mm Hg =
200 mm Hg
0.4 L
Jaques Charles
Charles invented the
hydrogen-filled
balloon and on
December 1, 1783,
he ascended into
the air and became
possibly the first
man in history to
witness a double
sunset
Jaques Charles
Jacques Charles carried out
experiments in 1786 and 1787
that showed a direct relationship
between the temperature and volume
of gases at constant pressure. You know that most
matter expands as its temperature rises. Gases are no
different. When Benjamin Thomson and Lord Kelvin
proposed an absolute
temperature scale in 1848,
it was possible to set up the
mathematical expression of
Charles’s law.
Charles’ Law: V and T
Charles’s law states that the volume of a
sample of gas is directly proportional to
the absolute temperature when pressure
remains constant.
Charles’ Law
Compares the V and T of
Confined gases
How does volume change with temperature?
D
V1 = 125 mL
V2 = 250 mL
T1 = 273 K
T2 = 546 K
1 mole of gas & Pressure = 753 mmHg
V and T Problem
A balloon has a volume of
785 mL on a Fall day when
the temperature is 21°C. In
the winter, the gas cools to
0°C. What is the new volume
of the balloon?
VT Calculation
Initial conditions
V1 = 785 mL
T1 = 21°C = 294 K
Final conditions
V2 = ?
T2 = 0°C = 273 K
785
V2 = _______
mL x __273
294
728.9 mL
K = _______
K
Check your answer: If temperature decreases,
V should decrease.
Charles Law
A sample of oxygen gas has a volume of
420 mL at a temperature of 18°C. What
temperature (in °C) is needed to change
the volume to 640 mL?
1) 443°C
2) 170°C
3) - 82°C
2) 170°C
Solution
A sample of oxygen gas has a volume of
420 mL at a temperature of 18°C. What
temperature (in °C) is needed to change
the volume to 640 mL?
T2 = V2T1
2) 170°C
V1
T2 = 640 mL x 291 K = 443 K
420 mL
T2 = 443 K - 273 K
= 170°C
Gay-Lussac’s Law
Joseph Louis Gay-Lussac in
1802 made reference in his
paper to unpublished work done by
Jacques Charles. Charles had found that
oxygen, nitrogen, hydrogen,
carbon dioxide, and plain air
expand to the same extent over
the same 80 degree interval.
Gay-Lussac
Gay-Lussac was no slouch
in the area of ballooning.
On September 16, 1804, he
ascended to an altitude of
7016 meters (just over
23,000 feet - about 4.3
miles). This remained the
world altitude record for
almost 50 years.
Gay-Lussac’s law:
The Pressure of a sample of gas at
constant volume will vary directly
with absolute (Kelvin) temperature.
Temperature & pressure
 As P then T
P1 P 2

T1 T 2
 At constant Volume & moles