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Gases

Any compound which occupies volume in nature is called as states of matter and is classified as Solids, Liquids and Gases. These states depend on their volume, temperature and pressure.

Definition: Gases are the molecules which move in all direction, having characteristics odour and has no define volume or shape.

Properties of Gases:
·  Easily expand and compress due to free movement of particles
·  Low density
·  High diffusion power

As compare to solid and liquids, the effect of changes in volume, temperature and pressure on definite quantity of gas is measurable. From the study of this relationship, the laws of gas are evolved.

Laws of Gas:
·  Boyle’s law
·  Charle’s law
·  Gay Lussac’s law
·  Dalton’s law
·  Graham’s law

Boyle’s Law
The volume of gas depends on its temperature and pressure. According to Boyle’s law, the volume (V) of a definite quantity of gas is inversely proportional to its pressure (P), at constant temperature (T).

V œ 1/P   (Temperature constant)
V = K/P   K = constant
PV = K

Let we consider the initial pressure of gas = P1
Initial volume of gas = V1
Final pressure of gas = P2
Final volume of gas = V2

P1V1 = P2V2


Charle’s Law
According to Charle’s Law, the volume (V) of a definite quantity of gas is directly proportional to its absolute temperature (T), at constant pressure (P).

V œ T  (Pressure constant)
V = KT  K = Constant
V/T = K
Similarly
V1/T1 = V2/T2
V1/V2 = T1/T2

Here we have temperature in a Kelvin temperature or absolute temperature. The temperature at which the volume of hypothetical gas will be zero is called as Kelvin temperature or absolute temperature. Kelvin has discovered this and the temperature is -273°C. The relation between Kelvin temperature (T) and Celsius temperature (t) is shown below.

T = t + 273

Gay Lussac’s Law
According to Gay Lussac’s law, the pressure of a gas of definite quantity at constant volume is directly proportional to absolute temperature.

P œ T
P = KT
P/T =K

P1/T1 = P2/T2
P1/P2 = T1/T2

A definite quantity of gas having volume (V1) at temperature (T1) and pressure (P1) changes to volume (V2), and the reaction is represented as below
 
P1V1T1 → P2VxT2 → P2V2T2

According to Boyle’s law
P1V1 = P2Vx
Vx = P1V1/ P2  ………………………………….(1)

According to Charle’s law
VxT1 = V2T2
Vx = V2T2/T……………………………………(2)

If we combine both the law, then as per equation (1) and (2)
P1 V1/ P2 = V2T2/T1
P1 V1/ T1 = P2V2/T2

PV/T = K
PV = KT
PV = nRT  

Where, K = changes if quantity of gas changes = nR
n = quantity of gas in mole
R = gas constant

Dalton’s Law
Dalton’s law is based on partial pressure of gas. Partial pressure is a sum of individual pressure of each gas in the gaseous mixture.

Consider one example:

A vessel contains a mixture of gas A and B having pressure of PA and PB respectively. According to Dalton’s law, the partial pressure of gaseous mixture is the sum of individual pressure of each gas.
P = PA + PB

Graham’s Law
According to Graham’s law, the rate of diffusion of various gases is inversely proportional to the square root of their densities, at constant temperature and pressure.

r œ 1/ (d)1/2
 
Where, r = rate of diffusion
  d = density of gas
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