1. Aqueous Solutions and the Concept of pH 19.2
Chemistry
Aqueous Solutions and the Concept of pH
19.2

2. Notation
A chemical formula written in square brackets represents the molar concentration of a solute.
[H+] is the molar concentration of hydrogen ion.
[NaOH] is the molar concentration of sodium hydroxide.

3. Water Water self ionizes to form H+ and OH- H2O(l) ↔ H+(aq) + OH-(aq)
The equilibrium constant becomes
Kw = [H+][OH-]
Equilibrium (ionization) Constant of Water
At 25C, Kw = 1.0 x 10-14
So for pure water [H+] = [OH-] = x
Then Kw = x2 = 1.0 x and x = 1.0 x 10-7

4. Water Solutions
A Neutral Solution contains equal amounts of H+ and OH-
[H+] and [OH-] are not usually equal in aqueous solutions
Based on Kw, [H+] and [OH-] are inversely proportional
When [H+] > [OH-] the solution is acidic
When [H+] < [OH-] the solution is basic

5. Sample Calculate the [H+] if [OH-] = 0.015 M. Kw = [H+][OH-]
So [H+] = Kw/[OH-]
[H+] = 1.0 x 10-14
[H+] = 6.7 x M
0.015

6. Try These Calculate the [OH-] when [H+] = 0.0026 M.
[OH-] = 1.0 x 10-14
What is the [H+] when [OH-] = 4.5 x 10-6 M?
[H+] = 1.0 x 10-14
[OH-] = 3.8 x M
0.0026
4.5 x 10-6
[H+] = 2.2 x 10-9 M

7. Try These Calculate the [OH-] when [H+] = 7.7 x 10-3 M 3.1 x 10-10 M

8. Try These Calculate the [H+] when [OH-] = 1.5 x 10-10 M 3.2 x 10-4 M

9. pH Another way to represent [H+] pH is often measured using a pH meter
[H+] = 10-pH  pH = -log10[H+]
As the pH changes by 1, [H+] changes by a power of 10
The lower the pH, the more acidic the solution
pH is often measured using a pH meter
Many natural compounds are pH sensitive and can be used to indicate various ranges of pH

10. pOH Similar to pH only it represents [OH-]
[OH-] = 10-pOH  pOH = -log10[OH-]
Since [H+][OH-] = 1.0x10-14, pH + pOH = 14

11. Sample pH = 0.60 pOH = 13.40 [H+] = 4.0 x 10-8M to 5.0 x 10-8M
Calculate
The pH and pOH when [H+] = 0.25M.
The [H+] in blood when the pH is 7.40 and At a pH of 7.30 a condition called acidosis develops.
pH = pOH = 13.40
[H+] = 4.0 x 10-8M to 5.0 x 10-8M

12. Try These Calculate the pH when 11.89 9.51 1.13 7.95
[H+] = 1.3 x M
[OH-] = 3.2 x 10-5 M
[H+] = 7.4 x 10-2 M
[OH-] = 8.9 x 10-7 M
11.89
9.51
1.13
7.95

13. Try These Calculate the pOH when 4.17 3.49 2.04 6.43
[OH-] = 6.8 x 10-5 M
[H+] = 3.1 x M
[OH-] = 9.1 x 10-3 M
[H+] = 2.7 x 10-8 M
4.17
3.49
2.04
6.43

14. Try These What is [H+] when pH = 2.59 pOH = 5.13 2.6 x 10-3 M

15. Try These What is [OH-] when pOH = 9.52 pH = 3.15 3.0 x 10-10 M

16. Indicators (HIn) Are weak acids
The acid is one color the conjugate base is another
The color is dependent on the ratio of the weak acid and its conjugate base

17. Sample
Thymol Blue’s acidic color red occurs at pH of 1 or less. Its basic color yellow occurs at pH 3 or greater What color are solutions with the following pHs?
pH = 10
pH = 7
pH = 2
pH = 1
Yellow
Yellow
Orange
Red