1. Acids
Lesson 9
Weak Acids
pH Calculations

2. 1. Calculate the pH of 0.45 M HCN.
You need an ICE chart for weak acids or bases!
HCN ⇄ H+ + CN-
I 0.45 M 0 0
C x x x
E x x x
Ka = [H+][CN-] = 4.9 x
[HCN]
Ka = x2 = 4.9 x x

3. The Ka is small, so x is small
We will find that x =
= 0.45
This means we can make the approximation that x = 0.45
We can do this anytime the Ka has an exponent of 10-4 or less
Ka = x2 = 4.9 x x
0 small ka

4. x2 = 4.9 x
0.45
x = [H+] = M
pH = -Log[ ]
pH = 4.83
2 sig figs due to molarity and Ka

5. 2. Calculate the pH of 0.60 M H3BO3
H3BO3 ⇄ H+ + H2BO3-
I 0.60 M 0 0
C x x x
E x x x
0 small ka
x2 = 7.3 x
0.60

6. x2 = 7.3 x
0.60
x = [H+] = x M
pH = -Log[2.09 x 10-5]
pH = 4.68
2 sig figs due to molarity and Ka

7. 3. Calculate the pH of a 0.20 M diprotic acid with a
Ka = x 10-7
H2X ⇄ H+ + HX-
I 0.20 M 0 0
C x x x
E x x x
0 small ka
x2 = 4.7 x 10-7
0.20

8. x = [H+] = x M
pH = -Log[ x 10-4]
pH = 3.51
2 sig figs due to molarity and Ka

9. 4. Calculate the pH of a saturated solution of Mg(OH)2.
This is a solubility equilibrium- no ICE
Mg(OH)2(s) ⇄ Mg2+ + 2OH-
s s 2s
Ksp = [Mg2+]][OH-]2 = x
[s][2s]2 = x
4s3 = x
s = x M

10. 2s = [OH-] = x M
pOH = 3.65
pH = 10.35