AP Chem Take out HW to be checked Today: Strong vs. Weak Base Calculations

Calculating pH of a base Calculations for bases work similarly to acids Strong bases ionize completely, so you know [OH-] Weak bases do not ionize completely, so you need to use Kb and an ICE table. Be careful! For bases, you know [OH-]. -log[OH-]=pOH, but most questions ask for pH

Strong Base Find the pH of 5.0 x 10-2 M NaOH. -log [5.0 x 10-2] pOH = 1.30 pH = 14-1.30 2 Ways [H+] = 1.0 x 10-14 5.0 x 10-2 -log [2.0 x 10-13] pH = 12.70

Be careful with group 2 hydroxides! pH of 3.0 M Ca(OH)2? [OH-] = 6.0 M (2 moles of OH- ions produced for every 1 mole of base) pOH = - 0.78 pH = 14.78

Weak Base Some don’t have OH- but still produce them. NH3(aq) + HOH(l) <==> NH4+(aq) + OH-(aq) Kb = [NH4+] [OH-] [NH3] If it makes OH-, you have to use Kb instead of Ka

Weak Base: Find the pH of 15 M NH3. Kb = 1.8 x 10-5 NH3(aq) + H2O (l) <==> NH4+(aq) + OH-(aq) I 15 M 0 0 C -x +x +x E 15-x +x +x 1.8 x 10-5 = x2 [15-x] x = [OH-] = 1.6 x 10-2 - log 1.6 x 10-2 = pOH = 1.80 pH = 14-1.80 = 12.20

Find the pH of 1.0 M methylamine. Kb = 4.38 x 10-4 CH3NH2 + H2O <==> CH3NH3+ + OH- I 1.0 M 0 0 C -x +x +x E 1.0-x +x +x 4.38 x 10-4 = x2 [1.0-x] x = [OH-] = 2.1 x 10-2 pOH = 1.68 pH = 14-1.68 = 12.32