Ostwald's Dilution Law
The relation showing how the degree of ionisation of a weak electrolyte increases on dilution.
Formula / equation
Ka = cα² / (1 − α)
Unit
mol/L
In depth
For small degrees of ionisation the law simplifies to α ≈ √(Ka/c), so ionisation is inversely proportional to the square root of concentration. This is why dilution increases the fraction ionised even though the total hydrogen ion concentration still falls.
Examples in the real world
Ethanoic acid is 1.3% ionised at 0.1 M but 4.2% ionised at 0.01 M.