$word_{count}$: 170

The common ion effect is Le Chatelier applied to ionic equilibria. Adding an ion already present shifts the equilibrium to reduce that ion's concentration. Applications: (1) Suppressing weak acid dissociation: adding CH3COONa to CH3COOH increases pH (less dissociation). (2) Reducing solubility: AgCl in NaCl solution has much lower solubility than in pure water. (3) Qualitative analysis: controlling [$S^{2-}$] by adjusting pH determines which metal sulfides precipitate. Quantitatively: for AgCl in $c_{common}$ M NaCl, [Ag+] = $\frac{Ksp}{c_common}$. The reduction factor = $\frac{c_common}{sqrt}$(Ksp). For 0.1 M NaCl: factor = 0.1/1.34 x 10^-5 = approximately 7400-fold reduction. The common ion effect is essential for understanding salt solubility control, buffer mechanism, and selective precipitation.