Ostwalds dilution law is a relationship proposed by Wilhelm Ostwald in 1888 between the dissociation constant Kd and the degree of dissociation α of a weak electrolyte. The law states that the degree of dissociation of a weak electrolyte is inversely proportional to the square root of the concentration, or the square root of the dilution. The law takes the form α = Kd/(1 + KdC), where α is the degree of dissociation, Kd is the dissociation constant, and C is the concentration of the electrolyte. The concentration of any one ionic species is given by the root of the product of the dissociation constant and the concentration of the electrolyte.

The Ostwald law of dilution provides a satisfactory description of the concentration dependence of the conductivity of weak electrolytes like CH3COOH and NH4OH. However, for strong electrolytes, the law fails badly since the supposed equilibrium constant is actually far from constant. This is because the dissociation of strong electrolytes into ions is essentially complete below a concentration threshold value. The decrease in molar conductivity as a function of concentration is actually due to attraction between ions of opposite charge as expressed in the Debye-Hückel-Onsager equation and later revisions.

In summary, Ostwalds dilution law is a relationship between the dissociation constant and the degree of dissociation of a weak electrolyte, which states that the degree of dissociation is inversely proportional to the square root of the concentration or the square root of the dilution.