Gas Laws

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I.
Pressure
force exerted on a unit area
aka: atmospheric pressure
air pressure
barometric pressure

  A. Standard Pressure
the accepted value for pressure under normal conditions at sea level.

· 1 atm (atmosphere)
· 101.3 kPa (kilopascal)
· 760 mmHg (millimeters mercury)
· 760 Torr

B. Normal Boiling/Freezing Points
temperature at which a substance boils/freezes at standard pressure.

C. STP
Standard Temperature and Pressure
(Listed in the Reference Tables)

Temperature 0oC or 273K
Pressure 101.3kPa or 1 atm

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II.
Kinetic Theory of Gases
Gases are made of separate, individual particles (molecules) in continuous motion
A. The theory is based on 5 assumptions
about an IDEAL GAS:
1. Gases are mostly empty space.
The volume of the molecules in a gas total a much smaller volume than the entire gas because there is so much empty space between the molecules.
2. Molecules in a gas are in constant, rapid, straight line motion.
3. Molecules experience perfectly elastic collisions.
A perfectly elastic collision is when no energy is
transfered between the two molecules that collide.
4. No forces exist between the molecules
There are no attractions or repulsions between molecules.
5. Molecules have different velocities (kinetic energies)
proportional to their temperature.
The molecules in a gas will move faster if
you raise the temperature of the sample.

B. Real vs. Ideal Gases
All of the gas laws are based on an ideal gas (meets the 5 criteria listed above).
Real gases only act ideal under the following conditions:
1. High Temperatures
The molecules will have high kinetic energy and move quickly.
2. Low Pressures
The molecules are far apart and are free to move around.
3. Low Mass
The molecules are take up less volume and move faster.

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III.
Vapor Concepts

A. Vapor-Liquid Equilibrium
The physical equilibrium (or balance) that exists between a
liquid and its vapor in a sealed container.
Molecules of liquid are continually vaporizing into a gas and
molecules of gas are continually condensing to liquid.
Equilibrium in an Erlenmeyer

B. Vapor Pressure
the pressure of a vapor in equilibrium with its liquid
Direct relationship graph
1. Temperature Dependent:
as temperature increases vapor pressure increases.
2. When vapor pressure = or > atmospheric pressure the liquid boils.
See reference table H

C. Dalton’s Law of Partial Pressure
the total pressure exerted by a gas is equal to the sum of the
pressures of the individual gases occupying the container.

PT = P1 + P2 + P3…+ Px

1. Used to correct the pressure of a gas when the gas is collected over water (gases are collected in water baths so that the gas can be seen in the form of bubbles and so that the gas is kept from spreading by the surrounding water pressure)

Pwet gas = Pdry gas + Pwater
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IV.
Gas Law Calculations
*Gas Laws require that temperature is in Kelvin*
K = oC + 273

A. Combined Gas Law
defines the relationship between pressure, volume and Kelvin Temperature.
P x V / T = k
where k = some unnamed constant
since this is true for many samples of a gas under different conditions

P1xV1/T1 = k and P2xV2/T2 = k

if k= k, which it must because the same letter must be the same number, therefor:
P1 x V1 = P2 x V2
T1
  
T2

B. Boyle’s Law
there is an inverse or indirect relationship between pressure and volume at constant temperature: as pressure increases volume decreases.
P x V = k
where k = some unnamed constant
Boyle's Law Indirect Relationship Graph
since this is true for many samples of a gas under different conditions

P1 x V1 = k and P2 x V2 = k

if k= k, which it must because the same letter must be the same number, therefor:
P1 x V1 = P2 x V2

C. Charles' Law
there is a direct relationship between volume and Kelvin temperature at constant pressure: as temperature increases volume increases.
V / T = k
where k = some unnamed constant
Charles' Law Direct Relationship Graph
since this is true for many samples of a gas under different conditions

V1/T1 = k and V2/T2 = k

if k= k, which it must because the same letter must be the same number, therefor:
V1 / T1 = V2 / T2

D. Gay-Lussac's Law
there is a direct relationship between pressure and Kelvin temperature at constant volume: as temperature increases pressure increases.
P / T = k
where k = some unnamed constant
Direct Relationship Graph
since this is true for many samples of a gas under different conditions

P1/T1 = k and P2/T2 = k

if k= k, which it must because the same letter must be the same number, therefor:

P1 / T1 = P2 / T2

E. Graham’s Law
The rate of diffusion of a gas is related to its mass.
The lighter the gas the faster it will move.
We will not be covering the formula or calculations that go with this law.
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Last revised May 9, 2007