Gas is one of the three forms of matter. Every known substance is either a solid, liquid or a gas. These forms differ in the way they fill space and change shape. A gas, such as air has neither a fixed shape nor a fixed volume and has weight.

1. The Theories and Behavior of
Gas
By Prof. Liwayway Memije-Cruz

2. Gas
Gas is one of the three
forms of matter. Every
known substance is either a
solid, liquid or a gas. These
forms differ in the way
they fill space and change
shape. A gas, such as air
has neither a fixed shape
nor a fixed volume and has
weight.

3. Properties of Gases

4. What makes a gas different from liquid and a
solid?
Most gases
exist as
molecules (in
case of inert
gases as
individual
atoms).

5. The molecules of gases are randomly
distributed and are far apart.
Gases can be easily
compressed, the
molecules can be
forced to be closed
together resulting to
lesser space between
them.

6. The volume or
space occupied by
the molecules
themselves is
negligible as
compared to the
total volume of the
container so that the
volume of the
container can be
taken as the volume
of the gas.

7. Gases have lower densities than
solids and liquids.

8. The attractive forces between molecules
(intermolecular) are negligible.

9. Most substances that are gaseous at normal
conditions have low molecular mass.

10. Measurable Properties of Gases

11. Note:
 1 atm = 1 atmosphere = 760 torr = 760 mm = 76
m Hg
 Temperature is always in Kelvin. At absolute zero
(0K) molecules stop moving entirely, the gas is as
cold as anything can get.
 Standard Temperature and Pressure (STP) or
Standard Conditions (SC):
 T = 0 0C = 273 0K
 P = 1 atm or its equivalents

12. Kinetic Molecular Theory
1. All matter is made of constantly moving atoms or molecules.
Because of their mass and velocity, they possess kinetic
energy, (K.E. = 1/2mv). The molecules collide with one
another and with the sides of the container.
2. There is no kinetic energy lost during collisions inspite of the
transfer of energy from one molecule to another. At any given
instant, the molecule do not have the same kinetic energy.
3. The average kinetic energy of the molecule is directly
proportional to the absolute temperature. At any given
temperature, the average kinetic energy is the same for the
molecules of all gases.

15. Boyle’s Law

16. Boyles Law
• At a given temperature, the volume occupied by a
gas is inversely proportional to the pressure.
Equation: P = k 1/v
Where:
P = pressure of a gas sample
V = volume of a gas sample
k = a constant
Therefore: PV = k

17. At a given temperature, the product of the pressure and
volume of a gas must be constant. If the pressure is
increased, the volume must decrease to maintain the
constant product. For a given gas sample to be studied
under different pressures, the following expressions
must hold:
P1V1 = P2V2
Where:
P1 = original pressure of a gas sample
V1 = original volume of the sample
P2 = new pressure of a gas sample
V2 = new volume of the sample

18. Example:
A sample of a gas entrapped in a cylinder with
a movable piston occupies a volume of 720 ml
under a pressure of 0.375 atm. What volume
will the gas occupy under a pressure of 1.000
atm when the temperature remains constant?
V1 = 720 ml P1 = 0.375 atm
V2 = ? P2 = 1.000 atm
V2 = 0.375 atm/1.000 atm
V2 = 270 ml

19. Boyle's Law states that at a given temperature, the
volume occupied by a gas is inversely proportional
to the pressure.

20. Exercise:
• A gas has a volume of 500 milliliters when
a pressure equivalent to 760 millimeters of
mercury is exerted upon it. Calculate the
volume if the pressure is reduced to 730
millimeters.

21. Charles Law
At a given pressure, the volume occupied by a gas is
directly proportional to the absolute temperature of the
gas.
Equation:
V = K T
Where:
V = volume of the gas sample
T = absolute temperature of the gas sample
K = a constant
Therefore: V/T = k

22. For a given sample, if the temperature is changed, this
ratio must remain constant, so the volume must change in
order to maintain the constant ratio. The ratio at a new
temperature must be the same as the ratio at the original
temperature, so:
V1 = V2 /T1 = T2
V1T2 = V2T1
Where:
V1 = original volume of sample of gas
T1 = original absolute temperature
V2 = new volume of the sample
T2 = new absolute temperature of the sample

24. Example:
A given mass of gas has a volume of 150 ml
at 25 0C. What volume will the sample of gas
occupy at 45 0C, when the pressure is held
constant?
V1 = 150 ml T1 = 25 + 273 = 298 0K
V2 = ? T2= 45 + 273 = 318 0K
V2 = 150 ml x 318 0K/2980K
V2 = 160 ml

25. Gay-Lussac’s Law
States that the pressure of a certain mass of gas is directly
proportional to its absolute temperature at constant volume.
P1 /T1 = P2/T2
Example:
An LPG tank registers a pressure of 120 atm at a temperature
of 27 0C. If the tank is placed in an air conditioned
compartment and cooled to 10 0C, what will be the new
pressure inside the tank?
• P1 = 120 atm T1 = 27 + 273 = 300 0K
• P2 = ? T2 = 10 + 273 = 283 0K
• P2 = 120 atm x 283 0K /2990K
• P2 = 113.6 atm

27. Combined Gas Law
States that the volume of a certain mass of gas is
inversely proportional to its pressure and directly
proportional to its absolute temperature.
A gas sample occupies 250mm at 27 0C, and 780
mm pressure. Find its volume at 0 0C and 760mm
pressure.
T1 = 270C + 273 = 300 0A
T2 = 00C + 273 = 273 0A
V2 = 250 mm x 2730A/3000A x 780 mm/760 mm
V2 = 234 mm

29. Ideal Gas Law
Follows the gas law perfectly. Such a gas is non-
existent, for no known gas obeys the gas laws at all
possible temperatures. There are two principal reasons
why real gases do not behave as ideal gases:
* The molecules of a real gas has mass, or weight, and
the matter thus contained in them cannot be destroyed.
* The molecules of a real gas occupy space, and thus
can be compressed only so far. Once the limit of
compression has been reached, neither increased
pressure nor cooling can further reduce the volume of
gas.