Define acids and bases according to Bronsted-Lowry theory. Derive relationship between pH and pOH.

Answer: pH + pOH = 14 (at 298 K)

Brønsted–Lowry theory: An acid is a substance that donates a proton (H\(^+\)) to another substance; a base is a substance that accepts a proton. e.g. in HCl + H\(_2\)O \(\rightarrow\) H\(_3\)O\(^+\) + Cl\(^-\), HCl is the acid and H\(_2\)O is the base.

Relation between pH and pOH: Water ionises as \(\mathrm{H_2O + H_2O \rightleftharpoons H_3O^+ + OH^-}\), and its ionic product is

\(K_w = [\mathrm{H_3O^+}][\mathrm{OH^-}] = 1.0\times10^{-14}\) at 298 K.

Taking logarithm: \(\log_{10}K_w = \log_{10}[\mathrm{H_3O^+}] + \log_{10}[\mathrm{OH^-}]\)

Multiplying by −1: \(-\log_{10}K_w = -\log_{10}[\mathrm{H_3O^+}] - \log_{10}[\mathrm{OH^-}]\)

\(\mathrm{p}K_w = \mathrm{pH} + \mathrm{pOH}\)

Since \(K_w = 10^{-14}\), \(\mathrm{p}K_w = 14\); \(\therefore\ \mathrm{pH} + \mathrm{pOH} = 14\).