Charles’ Law
Temperature
…is a measure of the average kinetic
energy of the molecules in a substance.
 Higher temperatures have faster
moving molecules.
 Colder temperatures have slower
moving molecules.

As gases cool…
Volume
When lines plotted for the cooling of gases are
extrapolated, the lines from all gases meet at -273°C.
(Assumes the gases do not liquefy.)
Absolute Zero
Absolute zero = -273.15°C
 The slowest a molecule could move is
stopped.
 The temperature at which all molecular
motion (in theory) stops (Ek = zero) is
absolute zero.
 Absolute zero has never been reached.

Absolute (Kelvin) Temperature Scale
Absolute zero (-273.15°C) is zero
degrees kelvin (0 K).
 There are no temperatures lower than
absolute zero, so degrees K never have
negative temperatures.

Comparing °C and K
-273.15°C
0°C
100°C
Absolute
zero
Water
Freezes
Water
Boils
0K
273.15 K
373.15 K
Converting:
°C to K: add 273.15.
 K to °C: subtract 273.15.

Example:
What is the boiling point of helium in K?
 b.p. = -269°C
 b.p. in K = -269 + 273.15 = 4.15 K

Charles’ Law

Volume of a gas is directly proportional
to the absolute temperature.
v α T
Charles’ Law equation
v1 v2
=
T1 T2
Assumes pressure is constant.
Temperature is absolute, in degrees K.
Example:

A 20.0 L volume of gas at SATP is
heated until the volume is 35.0 L. What
is the temperature, in °C, of the heated
gas?
Solution:
v1 v2
=
T1 T2
Note: T1 = 25°C = 298.15 K
20.0 L = 35.0 L
T2
298.15 K
T2 = 521.7625 K
T2 = 248.6125°C
T2 = 249°C
Your turn
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