1. NOTES: 19.2 – Hydrogen Ions & Acidity (pH and pOH)

3. pH scale:

4. Ionization of water ● Experiments have shown that pure water ionizes very slightly: H 2 O + H 2 O  H 3 O + + OH - ● Measurements show that in pure water: [H 3 O + ] = [OH - ] = 1 x 10 -7 M ● Pure water contains equal concentrations of H 3 O + and OH -, so it is NEUTRAL.

5. What is pH? ● pH is a measure of the concentration of hydronium ions in a solution. ● pH = -log [H 3 O + ] or ● pH = -log [H + ]

6. Example: What is the pH of a solution where [H 3 O + ] = 1 x 10 -7 M? pH = -log [H 3 O + ] pH = -log(1 x 10 -7 ) pH = 7

7. Example: What is the pH of a solution where [H 3 O + ] = 1 x 10 -5 M? pH = -log [H 3 O + ] pH = -log(1 x 10 -5 ) pH = 5 **When acid is added to water, the [H 3 O + ] increases, and the pH DECREASES.

8. Example: What is the pH of a solution where [H 3 O + ] = 1 x 10 -10 M? pH = -log [H 3 O + ] pH = -log(1 x 10 -10 ) pH = 10 **When a base is added to water, the [H 3 O + ] decreases, and the pH INCREASES.

9. The pH Scale Acid Neutral Base 0 7 14

15. What is pOH? ● pOH is a measure of the concentration of hydroxide ions (OH - ) in a solution. ● pOH = -log [OH - ]

16. Example: What is the pOH of a solution where [OH - ] = 1 x 10 -5 M? pOH = -log [OH - ] pOH = -log(1 x 10 -5 ) pOH = 5

17. How are pH and pOH related? ● At every pH, the following relationships hold true: [H 3 O + ] [OH - ] = 1 x 10 -14 M pH + pOH = 14

18. Example 1 : What is the pH of a solution where [H + ] = 3.4 x 10 -5 M? pH = -log [H + ] pH = -log(3.4 x 10 -5 M) pH = 4.5

19. Example 2 : What is the pOH of a solution where [OH - ] = 6.23 x 10 -8 M? pOH = -log [OH - ] pOH = -log (6.23 x 10 -8 M) pOH = 7.21

20. Example 3: The pH of a solution is measured to be 8.86. What is the [H + ] in this solution? pH = -log [H + ] 8.86 = -log [H + ] -8.86 = log [H + ] [H + ] = antilog (-8.86) [H + ] = 10 -8.86 [H + ] = 1.38 x 10 -9 M

21. Example 4: The pH of a solution is measured to be 1.65. What is the [OH - ] in this solution? pH + pOH = 14, so pOH = 14 – 1.65 = 12.35 pOH = -log [OH - ] 12.35 = -log [OH - ] -12.35 = log [OH - ] [OH - ] = antilog (-12.35) [OH - ] = 10 -12.35 [OH - ] = 4.467 x 10 -13 M

22. Example 5: What is the pH of a solution where [H + ] = 5.4 x 10 -6 M? pH = -log [H + ] pH = -log(5.4 x 10 -6 ) pH = 5.3

23. Example 6: What is the [OH - ] and pOH for the solution in example #5? [H 3 O + ][OH - ]= 1 x 10 -14 (5.4 x 10 -6 )[OH - ] = 1 x 10 -14 [OH - ] = 1.9 x 10 -9 M pOH = -log(1.9 x 10 -9 ) pOH = 8.7

24. Example 7: What is the pOH of a solution where [OH - ] = 1.47 x 10 -4 M? pOH = -log [OH - ] pOH = -log( 1.47 x 10 -4 ) pOH = 3.83

25. Example 8: What is the [H + ] and pH for the solution in example #7? [H 3 O + ][OH - ]= 1 x 10 -14 [H 3 O + ](1.47 x 10 -4 ) = 1 x 10 -14 [H 3 O + ] = 6.80 x 10 -11 M pH = -log(6.80 x 10 -11 ) pH = 10.2