Ostwald’s Dilution Law

Ostwald's dilution law explains equilibrium in aqueous electrolyte solutions based on Arrhenius' theory of electrolytic dissociation. For an electrolyte AB at concentration c, the dissociation constant K equals cα²/(1−α), where α is the degree of dissociation. This law describes how weak electrolytes like acetic acid and NH₄OH reversibly dissociate into ions at equilibrium.

Strong electrolytes like HCl and NaF are almost completely dissociated, with α approaching one, making (1−α) approach zero. This renders the law mathematically invalid. Water's high dielectric constant weakens electrostatic forces, causing near-complete dissociation and eliminating the ion-molecule equilibrium that the law requires.

For an electrolyte AB with concentration c and degree of dissociation α, each ion has equilibrium concentration cα, while the undissociated fraction has concentration c(1−α). These concentrations define the dissociation constant K, which governs the equilibrium state of weak electrolyte solutions.

Weak electrolytes like acetic acid partially dissociate, with α significantly less than one, allowing Ostwald's dilution law to apply. Strong electrolytes like NaCl and NaF are electrovalent compounds formed by electron transfer; they dissociate almost completely with α near one, making the law inapplicable to them.

According to Coulomb's law, the dielectric constant of the medium affects electrostatic force strength and conductivity. Water's high dielectric constant weakens electrostatic forces between ions, promoting near-complete dissociation of strong electrolytes and enabling ion mobility for electrical conduction in solutions.

For very weak electrolytes where α is much less than one, the term (1−α) approximates to one. The dissociation constant simplifies to K ≈ cα², making calculations more straightforward for solutions with minimal dissociation and allowing easier prediction of equilibrium behavior.

Strong electrolytes like NaCl and NaF are electrovalent compounds composed solely of ions in their crystal structure, not molecules. When dissolved in water, these ions gain mobility and conduct electricity. Weak electrolytes exist as molecules that partially dissociate into ions upon dissolution in solution.