1. Acids
Lesson 13
pH for
Weak Bases

2. NH3 + H2O ⇄ NH4+ + OH- I 0.50 M 0 0 C x x x E 0.50 - x x x Small Kb
Calculate the pH of 0.50 M NH3.
Weak Base Need Kb(NH3) = Kw
Ka(NH4+)
= 1 x
5.6 x = x 10-5
NH H2O ⇄ NH4+ + OH-
I 0.50 M
C x x x
E x x x
Small Kb

3. Kb = [NH4+][OH-]
[NH3]
= x2 = x 10-5
0.50
x = M
pOH = 2.52
pH =

4. 2. Calculate the pH of 0.20 M Na2CO3.
Weak Base Need Kb(CO32-) = Kw
Ka(HCO3-)
= 1 x
5.6 x
= x 10-4
CO H2O ⇄ HCO3- + OH-
I 0.20 M
C x x x
E x x x
Small Kb

5. Kb = [HCO3-[OH-]
[CO32-]
= x2 = x 10-4
0.20
x = M
pOH =
pH =

6. 3. Calculate the pH of 0.20 M NaCl.
NaCl is a neutral salt
pH = 7.00

7. 4. 15.0 mL of 0.20 M H2SO4 reacts with 40.0 mL of
0.20 M KOH, calculate the pH of the resulting solution.
1H2SO KOH → K2SO HOH
L x mole L x mole
1 L 1 L
I moles moles
C moles moles
E moles

8. final volume = 15.0 mL + 40.0 mL = 55.0 mL
[KOH] = moles = M L
pOH = pH = 12.56