Graham Law of Diffusion

Graham Law of Diffusion:

After comparing the rates of diffusion for different gases, Graham found that under similar conditions of temperature and pressure, the rates of diffusion of gases are inversely proportional to the square roots of their densities. This is called Graham’s law of diffusion. Mathematically, it can be expressed as-

r1/r2 = √d2/d1 ………………(I)

Where, r1 and r2 are the rates of diffusion of two gases and d1 and d2 stand for their densities.

Rate of diffusion = Volume diffused/Time taken

Noe if V1 is the volume of a certain gas diffusing in time t1 and V2 is the volume of another gas diffusing in time t2, then we have-

r1 = V1/t1 and r2 = V2/t2

Thus,

r1/r2 = V1/t1 / V2/t2 = √d2/d1 ………………….(II)

When volume is the same, i.e., V1 = V2, the above relation becomes-

t2/t1 = √d2/d1 or t1/t2 = √d1/d2 …………………..(III)

If different volumes, V1 and V2, of two gases diffuse in same time t, then we have,

r1/r2 = V1/t / V2/t = √d2/d1 or V1/V2 = √d2/d1

Since, molecular weight = 2 x Vapour density, Graham’s law equation (I) can be written as-

r1/r2 = √d2/d1 = √2d2/2d1 = √M2/M1
i.e. r1/r2 = √M2/M1

Where, r1 and r2 are the rates of diffusion and M1 and M2 the molecular weights of two gases, respectively.

Thus, the rates of diffusion of two gases are inversely proportional to the square roots of their miolecular weights.

Graham law of diffusion find application-

  • In the separation of gases from their mixture.
  • In the determination of the densities and molecular weights of gases.

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Avogadro LawDalton Atomic Theory
Electrical Properties of SolidsSecond and Third Law of Thermodynamics
Radioactive Disintegration SeriesAtomic Structure– Tamil Board

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