Ostwald’s Dilution Law

Ostwald’s Dilution Law

The ideas of Arrhenius were expressed quantitatively by Ostwald in terms of a dilution law. Consider a binary electrolyte AB that exists in solution partly as the undissociated species AB and partly as the ions A⁺ and B^-. Let the concentration of AB be C. If  is the fraction of AB that has ionised, the concentration of un-ionised AB is C (1-) and the concentration of K and B ions is for each ion. Thus;

AB  A⁺ + B^-                                                                                                                  (13)

C (1-)   C + C

By the law of mass action the equilibrium constant, K_eq is given by,

K_eq = [A⁺] [B^-]                                                                                                             (14)

            [AB]

In terms of and c

K_eq =                                                                                                    (15)

Equation 15 is the mathematical expression of Ostwald’s Dilution Law.

If in eqn. 15 we set = _m/_m

Then

K_eq = (m/_m )²C

        (1-m/_m )

            = _____cm ²                    

            _m ² (1-m/_m )

On rearrangement,

c m = K_eq (  -_{m )}

                                                                                    (16)

A plot of cm against 1/m should give a straight line of

Slope.= K_{eq m} ² and intercept = -K_{eq m} The equilibrium constant may thus be determined.

The Arrhenius theory and hence the Ostwald’s dilution aw gives satisfactory results for weak electrolytes but not for strong electrolytes. Weak electrolytes give constant values of K which means the values of a derived from measurement of week electrolytes are correct.