Skip to main content
9.S: The Gaseous State (Summary)
-
-
Last updated
-
-
Save as PDF
-
-
Gasses are
compressible
because there is so much space between individual gas particles. Energy transferred from the collision of gas particles with their container exerts a
gas pressure
. In chemistry, we typically measure gas pressure using units of
atmospheres
(atm) or in
mm Hg
(also referred to as torr). One atmosphere of pressure equals exactly 760 mm Hg.
-
The volume of a gas varies
inversely
with the applied pressure; the greater the pressure, the smaller the volume. This is relationship is referred to as
Boyles’s Law
. For a two-state system where the number of moles of gas and the temperature remain constant, Boyle’s Law can be expressed as
\[P_{1}V_{1}=P_{2}V_{2} \nonumber \]
-
The volume of a gas varies
directly
with the absolute temperature; the higher the temperature, the larger the volume. This is relationship is referred to as
Charles’s Law
. For a two-state system where the number of moles of gas and the pressure remain constant, Charles’s Law can be expressed as: \[\frac{V_{1}}{T_{1}}=\frac{V_{2}}{T_{2}} \nonumber \]
-
In this equation, the
absolute temperature
in Kelvin (K) must be used. Kelvin is defined as (degrees centigrade + 273.15). Zero degrees Kelvin is referred to as “absolute zero” and it is the temperature at which (theoretically) all molecular motion would cease.
-
The volume of a gas varies
directly
with the number of moles of the gas that are present; the greater the number of moles, the larger the volume. This is relationship is referred to as
Avogadro’s Law
. For a two-state system where the temperature and the pressure of a gas remain constant, Avogadro’s Law can be expressed as: \[\frac{V_{1}}{n_{1}}=\frac{V_{2}}{n_{2}} \nonumber \]
-
Because the volume of a gas varies directly with the number of moles of the gas that are present and with the absolute temperature (in Kelvin), and inversely with the pressure, the gas laws can be combined into a single proportionality; \[V\propto \left ( \frac{nT}{P} \right ) \nonumber \]
-
This proportionality can be converted to an equality by inserting the proportionality constant
R
(the universal gas constant), where
R
= 0.082057 L atm mol
-1
K
-1
, and can be re-written as: \[V=R\left ( \frac{nT}{P} \right )\; \; or\; \; PV=nRT \nonumber \]
-
This is referred to as the Ideal
Gas Law
and is valid for most gasses at low concentrations. For a two-state system where the identity of the gas does not change, the Ideal Gas Law can be expressed as: \[\frac{P_{1}V_{1}}{n_{1}T_{1}}=\frac{P_{2}V_{2}}{n_{2}T_{2}} \nonumber \]
-
The gas constant
R,
is calculated based on the experimental observation that
exactly
one mole of any gas at
exactly
1 atm and at 0 ˚C (273 K) has a volume of
22.414 L
. The conditions, 1 atm and 0 ˚C, are called standard temperature and pressure, or
STP
.
-
The ideal gas laws allow a quantitative analysis of whole spectrum of chemical reactions involving gasses. When you are approaching these problems, remember to
first
decide on the
class
of the problem:
-
If it is a “single state” problem (a gas is produced at a single, given, set of conditions), then you want to use
PV
= nRT.
-
If it is a “two state” problem (a gas is changed from one set of conditions to another) you want to use \[\frac{P_{1}V_{1}}{n_{1}T_{1}}=\frac{P_{2}V_{2}}{n_{2}T_{2}} \nonumber \]
-
If the volume of gas is quoted at STP, you can quickly convert this volume into moles with by dividing by 22.414 L mol
-1
.