← Equilibrium principles in aqueous systems
3 Water, Kw and pH
The essentials
Ionic product of water K_w=[\ce{H+}][\ce{OH-}]=1.0\times10^{-14}\ \text{(25 °C)} Always true in aqueous solution. Increases with temperature (self-ionisation is endothermic). pH and pOH \mathrm{pH}=-\log[\ce{H+}]\qquad[\ce{H+}]=10^{-\mathrm{pH}}\qquad \mathrm{pH}+\mathrm{pOH}=14 Neutral at 25 °C: pH 7. Acidic pH < 7, basic pH > 7. Worked example pH of a strong base Find the pH of 0.050 mol L⁻¹ NaOH. NaOH is strong ⟹ [OH⁻] = 0.050 mol L⁻¹. [H⁺] = K
w/[OH⁻] = 1.0×10⁻¹⁴ / 0.050 = 2.0×10⁻¹³. pH = −log(2.0×10⁻¹³) = 12.7. ✓ pH = 12.7 (or: pOH = −log0.050 = 1.3, pH = 14 − 1.3 = 12.7).