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

2. 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

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

4. 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 =
pH = 14.78

5. 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

6. 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 log 1.6 x 10-2 = pOH = 1.80 pH = = 12.20

7. Find the pH of 1.0 M methylamine. Kb = 4.38 x 10-4
CH3NH2 + H2O <==> CH3NH OH-
I 1.0 M
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 = = 12.32