Gas Law



Boyle’s law

  • At constant temperature, the pressure of a fixed amount (i.e., number of moles n) of gas varies inversely with its volume. This is known as Boyle’s law.

  • If a fixed amount of gas at constant temperature T occupying volume V1 at pressure p1 undergoes expansion, so that volume becomes V2 and pressure becomes p2, then according to Boyle’s law :

  • Experiments of Boyle, in a quantitative manner prove that gases are highly compressible.
  • Also , With the help of Boyle’s Law , we determine that


  • This shows that at a constant temperature, pressure is directly proportional to the density of a fixed mass of the gas.




Charles law 

  • Charles’ law, states that pressure remaining constant, the volume of a fixed mass of a gas is directly proportional to its absolute temperature.

Graph -

  • Charles found that for all gases, at any given pressure, graph of volume vs temperature (in Celsius) is a straight line and on extending to zero volume, each line intercepts the temperature axis at – 273.15 ° C.
  • Slopes of lines obtained at different pressure are different but at zero  volume all the lines meet the temperature axis at – 273.15 ° C

Absolute Zero –

  • The lowest hypothetical or imaginary temperature at which gases are supposed to occupy zero volume is called Absolute zero.

Gay Lussac’s Law

  • It states that at constant volume, pressure of a fixed amount of a gas varies directly with the temperature.

Graph –


Avogadro’s law

  • It states that equal volumes of all gases under the same conditions of temperature and pressure contain equal number of molecules.
  • If the amount of gas increases, then so does its volume. 
  • V = k4n
  • The number of molecules in one mole of a gas has been determined to be 6.022 X 1023 and is known as Avogadro constant.

Standard temperature and pressure (STP)

  • Standard temperature and pressure means 273.15 K (0°C) temperature and 1 bar (i.e., exactly 105 Pascal) pressure.
  • Volume of 1 mole of any gas at STP is 22.4 L

 Graham’s Law 

  • According to this law, the rate of diffusion of a gas is inversely proportional to the square root of its density.

Since molecular weight of gas is equal to twice the vapor density

  • Using equation 2 in equation 1 , we get
  • Hence, Graham’s Law of diffusion can also be stated as the rate of diffusion of gases is inversely proportional to the square root of their molecular masses.