1. Acids
Lesson 8
Weak Acids
pH, Ka Calculations

2. Weak Acids calculations
Helpful Hints:
Weak acids do NOT ionize 100%
There is an equilibrium state
NEED….ICE tables!
Ka can be used directly from table!

3. Weak Acids calculations
Two types of questions -calculate pH, or [H3O+] -calculate Ka

4. Calculating pH or [H3O+]

5. 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

6. 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

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

8. “Percentage dissociation” is not more than 5%
what you just learnt:
You need to ALWAYS state this assumption of x being insignificant, otherwise it is considered a chemical error not to state it.
You can use it anytime Ka has an exponent of 10-4 or less
In this course, we will always make that assumption just be sure to state it!
There is another way you can also justify it:
“Percentage dissociation” is not more than 5%

9. “Percentage dissociation” is not more than 5%
Refers to the amount of [H3O+] produced as a percentage of the original amount of acid present.
% dissociation = [H3O+] found at equilibrium x 100%
[HA] at start

10. 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
Anytime there is a polyprotic acid, only use the first proton for the ionization.
0 small ka
x2 = 7.3 x
0.60

11. 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

12. 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

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

14. What if asked to calculate Ka?

15. Ex: 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]

16. TRY: The pH of a 1. 0 M triprotic weak acid is 4. 568
TRY: The pH of a 1.0 M triprotic weak acid is Calculate the Ka and identify the acid.

17. TRY: The pH of a 1. 0 M triprotic weak acid is 4. 568
TRY: 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

18. TRY: The pH of a 0. 010 M solution of HBr is 2. 00
TRY: The pH of a M solution of HBr is What is the Ka for HBr?

19. Another example
Calculate the pH of 0.30 M solution of NH4NO3
79 p152

21. Homework
Read Pages Go through examples Pg 152 #74-81