1. Acids
Lesson 10
Calculating Ka
From pH

2. 1. The pH of 0.100 M H2C2O4 is 1.28. Calculate the Ka for the weak acid.
[H+] = M
H2C2O4 ⇄ H+ + HC2O4- I
0.100
0 0 C E
[H+][HC2O4-]
Ka =
( )2 =
= x 10-2
[H2C2O4]

3. 2. If the pH of 0.40 M NH3 @ 25 oC is 11.427, calculate the Kb.
pOH =
[OH-] =
[OH-] = M
NH3 + H2O ⇄ NH4+ + OH- I C E
0.3973
[NH4+][OH-]
( )2
= x 10-5
Kb = =
0.3973
[NH3]

4. 3. The pH of a 1. 0 M triprotic weak acid is 4. 568
3. The pH of a 1.0 M triprotic weak acid is Calculate the Ka and identify the acid.
Ka = 7.3 x
Hint- it’s not very exciting
Boric acid

5. 4. The pH of 0.20 M NaCN is 11.456, calculate the Kb for CN-.
CN- + H2O ⇄ HCN OH- I C E
pH =
pOH = 2.544
[OH-] = M
[HCN][OH-]
( )2
= x 10-5
Kb = =
[CN-]
0.1971

6. 5. If the pH of H3BO3 is 5.42, calculate the initial concentration.
[H+] = x M
H3BO ⇄ H H2BO3- x I C
x 10-6
x 10-6
x 10-6 E
x x 10-6
x 10-6
x 10-6
0 small ka
(3.802 x 10-6)2 =
7.3 x x x =
0.020M

7. Calculate the pH of a solution made by mixing 100. 0 mL of 0
Calculate the pH of a solution made by mixing mL of M HCl with 100 mL of water.
HCl → H+ + Cl-
1( ) 2
0.050 M
0.025 M M
pH = 1.60