– Benzoic Acid (C H COOH) . × – Hypochlorous Acid (HCIO) . × – Hydrocyanic Acid (HCN) . × – Phenol (C H OH) .
× – 2H O (l) H O + (aq) + OH – (aq) K w = [OH – ][H O + ] = – Let, x = [OH – ] = [H O + ] from H O. The H O + concentration is generated (i) from the ionization of HCl dissolved i.e., HCl(aq) + H O(l) H O + (aq) + Cl – (aq), and (ii) from ionization of H O. In these very dilute solutions, both sources of H O + must be considered: [H O + ] = – + x K w = ( – + x)(x) = – or x + – x – – = [OH – ] = x = . × – So, pOH = .
and pH = . . . Ionization Constants of Weak Acids Consider a weak acid HX that is partially ionized in the aqueous solution.
The equilibrium can be expressed by: HX(aq) + H O(l) H O + (aq) + X – (aq) Initial concentration (M) c Let α be the extent of ionization Change (M) -c α +c α +c α Equilibrium concentration (M) c-c α c α c α Here, c = initial concentration of the undissociated acid, HX at time, t = . α = extent up to which HX is ionized into ions. Using these notations, we can derive the equilibrium constant for the above discussed acid-dissociation equilibrium: K a = c α / c( - α ) = c α / - α K a is called the dissociation or ionization constant of acid HX. It can be represented alternatively in terms of molar concentration as follows, K a = [H + ][X – ] / [HX] ( .
) At a given temperature T ,