Gas Laws

All gases usually show the same behaviour when conditions are normal. But with small changes in body conditions such as pressure, temperature or volume this indicates deviation. The rules of gas are to analyse this gas behaviour. Flexible conditions such as pressure, volume and temperature of a gas indicate its true nature. Which is why gas laws are the relationship between these variables.

Gas laws give the relations between pressure, volume and temperature of a given gas. The individual gas laws are described hereunder as follows:

• Boyle’s Law: It describes the relationship between the pressure and the volume of a gas.
• Charles’s Law: It describes the relationship between the volume occupied by a gas and the absolute temperature.
• Gay-Lussac’s Law: It describes the relationship between the pressure exerted by a gas on the walls of its container and the absolute temperature associated with the gas.
• Avogadro’s Law: It describes the relationship between the volume occupied by a gas and the amount of gaseous substance.

The Combined Gas Law (or the Ideal Gas Law) can be obtained by combining the four laws listed above. All these laws are only valid for ideal gases. In other words, we can define that the gases which obey the gas laws are called ideal gases. 

Boyle’s law

For a given mole of gas at a constant temperature, the pressure of the given gas is inversely proportional to the volume of that gas.

V ∝ 1/P

PV = K1

Where, 

V is the volume of the gas

P is the pressure of the gas 

K1 is the constant

In another form, we can write it as, P1V1 = P2V2

Here. P1 is the initial pressure, V1 is the initial volume, P2 is the final pressure, and V2 is the final volume.

Charles’ Law

It relates the volume and temperature of the given mass of gas at constant pressure.

Statement: “The volume of a given amount of gas at a constant pressure varies as per its absolute temperature.”

Mathematically, we can state that-

V ∝ T

or  V = KT 

or V/T = K = constant 

or it can be written as V1 / T1 = V2 / T2

Here, the temperature is in Kelvin. It is called the absolute scale of temperature. This temperature scale was introduced by the British physicist and mathematician Lord Kelvin. The lower limit of the scale is called absolute zero, which corresponds to -273°C. At absolute zero or -273°C, the molecular motion of the gas stops and the volume of the gas becomes zero. The gas becomes liquid or solid. Thus, absolute zero is that temperature at which no substance exists in the gaseous state. The temperature is always obtained by adding 273 to the temperature expressed in degrees celsius.

Mathematically, K = (ºC+273)

This absolute scale of the temperature is used in gas laws. 

Gay-Lussac’s Law

It relates the pressure and the absolute temperature of the given mass of gas at constant volume. 

Statement: Volume remains constant. The pressure of a given mass of a gas increases or decreases by 1273 of its pressure at 0°C per degree change in temperature. 

P=P0(1+t273)

Or P=P0(273+t273)

Or P=P0(TT0)

Or PT=P0T0

Or PT=constant 

At constant volume, the pressure of the given amount of gas is directly proportional to the absolute temperature. 

Avogadro’s Law

Statement: Equal volume of all the gases contain equal no of molecules under identical temperature and pressure conditions.  

One mole of gas (6.0231023) molecules under STP occupy 22.4L of volume. 

The molar volume of a gas at a given temperature and pressure is a specific constant. It is independent of the gas taken.

Thus, it can be written as V∝n ( At constant temperature and pressure)

                               Or V1n1=v2n2

At SATP (standard ambient temperature and pressure) means 25°C and 1 bar pressure, molal volume =24800ml.

Ideal Gas Equation

The ideal gas law, also called the general gas equation, depicts the state of a hypothetical ideal gas. The equation is a good approximation of the behaviour of many gases under different conditions, although it has several limitations. The ideal gas law was first stated by Benoît Paul Émile Clapeyron in 1834. It combines the empirical Avogadro’s law,  Boyle’s law, Charles’s law, and Gay-Lussac’s law. The ideal gas law is also written in an empirical form as mentioned below:

PV=nRT

Where, P= pressure

V= volume

n= no of mole 

R= Ideal gas constant

The ideal gas constant is the same for all gases but can vary based on which units are being used; the most common expressions are R = 0.0821 (L*atm/mol*K) or ‘R=8.31 (J/mol*K).

Conclusion

The gas rules above give us an indication of the various gas properties of the gas with the changing temperature, pressure and magnitude. Created in the early 17th century, gas laws were in place to assist scientists in determining volume, quantity, pressure, and temperature when it came to gas. Boyle’s law tells us that gas volume increases as pressure decreases. Charles’ Law tells us that gas volume rises as temperatures rise. And the Avogadro Act tells us that gas volume goes up as the price of gas goes up. A proper gas law is a combination of three simple gas laws.