Law of dilution

Wilhelm Ostwald’s dilution law is a relationship proposed in 1888 between the dissociation constant Kd and the degree of dissociation α of a weak electrolyte. The law takes the form Wilhelm Ostwald’s dilution law is a relationship proposed in 1888 between the dissociation constant Kd and the degree of dissociation α of a weak electrolyte. The law takes the form Where the square brackets denote concentration, and c0 is the total concentration of electrolyte. Using α = Λ c / Λ 0 {displaystyle alpha =Lambda _{c}/Lambda _{0}} , where Λ c {displaystyle Lambda _{c}} is the molar conductivity at concentration c and Λ 0 {displaystyle Lambda _{0}} is the limiting value of molar conductivity extrapolated to zero concentration or infinite dilution, this results in the following relation: Consider a binary electrolyte AB which dissociates reversibly into A+ and B− ions. Ostwald noted that the law of mass action can be applied to such systems as dissociating electrolytes. The equilibrium state is represented by the equation: If α is the fraction of dissociated electrolyte, then αc0 is the concentration of each ionic species. (1 - α) must, therefore be the fraction of undissociated electrolyte, and (1 - α)c0 the concentration of same. The dissociation constant may therefore be given as For very weak electrolytes (however, neglecting 'α' for most weak electrolytes yields counterproductive result) α ≪ 1 {displaystyle alpha ll 1} , implying that (1 - α) ≈ 1.