Ostwald Dilution Law

Ostwald dilution law

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Ostwald dilution law is an application of law of mass action for dissociation equilibrium of weak electrolyte. Consider a weak binary electrolyte,

AB   <——>    A⁺        +       B^–

(Weak Electrolyte)

Moles before dissociation (t=0)                                  1                      0                      0

Moles after dissociation (At time ‘t’)                          1-α                   α                      α

α is degree of dissociation of weak electrolyte ‘AB’ & ‘C’ is the concentration of AB in mole/litre then

[AB] = C(1–α)

[A⁺] = Cα

[B^–]  = Cα

At equilibrium

kα = [A⁺] [B⁺]/[AB] = (Cα)*(Cα)/C(1-α)

kα =Cα*α/(1-α)

Ka= dissociation constt. of ‘AB’

For weak electrolyte, α<<<1

So 1–α

Ka=Cα²

α=ka/c =ka.V

“Degree of dissociation of weak electrolyte is directly proportional to square root of dilution.”

Limitations of ostwald Dilution Law-

1. This law is applicable only for weak electrolyte
2. This law is not applicable for strong electrolytes because strong electrolytes completely ionize at all dilution.