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What is Graham’s Law?

Chemistry | 7-14 yrs | Interactive

What is Graham’s Law of Diffusion and Effusion?

In1829, Thomas Graham, a Scottish Chemist formulated the Graham’s Law of the Diffusion and Effusion of Gases. According to this Law, the rate of Diffusion of different gases, at a constant temperature, is inversely proportional to the square root of its density.

Formula for Graham’s Law of Diffusion and Effusion

r ∝ 1/(M)½

where,

r = rate of diffusion or effusion
M = molar mass

The Molar Mass tells you the number of grams per mole of a compound. The units for molar mass is grams/mole. It indicates the number of atoms, ions, molecules, or formula units present in a particular chemical.

3 Fun facts about Graham’s Law

  1. Our noses can smell because of Graham’s Law. Graham’s Law determines what we smell faster. Some gases travel faster and some gases travel slower.
  2. Respiration in all living things is based on Graham’s Law, including respiration through skin or lungs.
  3. You would have noticed balloons lose air from them slowly, even if they haven’t burst? This is because of Diffusion of Gases.

What is the Ideal or Universal Gas Law?

According to the Universal or Ideal Gas Law, the relationship between the pressure, volume, and temperature for a fixed mass (quantity) of gas is as follows:

  1. If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas.
  2. If the temperature and volume remain constant, then the pressure of the gas changes directly proportional to the number of molecules of gas present.
  3. If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume.
  4. If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.

Formula for the Universal or Ideal Gas Law

The formula for the Universal Gas or the Ideal Gas Law is :

PV = nRT

where,

p is pressure
V is volume
n is the number of moles
R is the universal gas constant
T is temperature (K)

The Universal or Ideal Gas Law can be derived from the Kinetic Molecular Theory.

What is Kinetic Molecular Theory?

Kinetic Molecular Theory shows us how individual gas molecules behave or interact with one another.

According to the Kinetic Molecular Theory :

  1. Molecules are point masses (they have no volume)
  2. Gas molecules exert no force on each other unless they collide
  3. Collisions of molecules with each other or the walls of the container do not decrease the energy of the system
  4. The molecules of a gas are in constant and random motion
  5. The temperature of a gas depends on its average kinetic energy, that is, the energy of an ideal gas is entirely kinetic.
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