pH of Bases

Determine if acid or alkali.
 Check if weak or strong before doing method.
 Also always check if 2OH or 2H as would be dibasic and would need to multiply conc x 2.
 Check in questions if given pKa or Ka.
 Kw: Ionic product of water- the ions in water H+ and OH- multiplied together. Kw increases with
temperature.
[H+ (aq)] [OH- (aq)] = 1.00 x 10-14 mol2dm-6 = (Kw)
at 298K.
 Water: Water produces same number of H+ and OH-, [H+ (aq)] = [OH- (aq)] so pH = 7.
– So Kw = [H+]2
.
 Acids and Alkalis: When [H+ (aq)] > [OH- (aq)] solution is acidic. When [H+ (aq)] < [OH- (aq)]
solution is alkaline.
 Dissociation of water is endothermic because Kw increases with temperature OR dissociation
involves bond breaking.

At different temperature, pH 7 is not neutral but can be acidic or alkaline.
 pH of Strong Bases: An alkali is a soluble base than completely dissociates to release OH- ions in
aqueous solution. E.g. NaOH -> Na+ + OH-. NaOH is monobasic base as each mole of NaOH
releases one mole of OH- ions.
 Calculate pH of Strong Base: First write [NaOH(aq)] = [OH- (aq)].
 Dibasic: Each mole of alkali, releases two moles of [OH- (aq)]. For example Mg(OH)2 or Ca(OH)2.
So first write [NaOH(aq)] = [OH- (aq)] x 2.
 pH of Weak Bases: E.g. ammonia gas. Equilibrium positioned well to the left, towards
undissociated alkali. One mole of weak base, releases far less than one mole of OH- ions. Use
same method to find pH of weak bases as weak acids.