1. CHAPTER 19 NOTES:
Part III: pH and [H+]

2. Acids and Bases HA  H+ + A- MOH  M+ + OH-
Working with the Arrhenius acid definition, we say that acids produce hydrogen ions:
HA  H+ + A-
acid hydrogen ion anion
And bases produce hydroxide ions:
MOH  M OH-
base cation hydroxide

3. Water, being made up of both H+ and OH- can dissociate giving us the formula:
H2O  H+ + OH-
But it only does this once out of every 500,000,000 molecules!

4. H2O
We can measure how many H+ ions and OH- ions there are in regular water.
[H+]=1x10-7M and [OH-]=1x10-7M
From this information, we can get a dissociation constant for water.

5. Kw = dissociation constant
Kw =[H+]x[OH-]
Kw =(1.0x10-7M)x(1.0x10-7M)
Kw = 1.0x10-14

6. ACIDS, BASES and CONCENTRATION
Even if a solution is acidic or basic,
[H+][OH-] = 1x10-14
Like a see-saw, raising or lowering the amount of [H+] will change the amount of [OH-] in solution

7. [H+] and [OH-] Concentration
Which solution is neutral?
Which solution is acidic?
Which solution is basic?

8. THE pH SCALE The pH scale is a logarithmic scale from 0 to 14.
logarithmic scale…each step is ten times stronger/weaker than the one next to it!
It measures the concentration of H+ (pH stands for “power of hydrogen”) with the formula:
pH = -log[H+]

9. Water has a pH of 7.0, which is neutral.
(same number of H+ and OH- ions)
Less than 7.0 is acidic
(the closer to zero the stronger the acid).
More than 7.0 is basic
(the closer to 14 the stronger the base).

10. Concentration and pH
Another way to look at it is the concentration of hydrogen in a solution is 1x10-a M where “a” is the pH value.

11. THE pOH SCALE
The pOH scale measures the hydroxide ion concentration.
Kw=[H+][OH-] = 1x10-14
14 = pH + pOH
Therefore, you can solve for pH, pOH and [OH-] given just [H+].

12. pH/pOH SQUARE
[H+] = 10-pH
[OH-] = 10-pOH
pOH= -log[OH-]
pH= -log [H]

13. HOW TO MEASURE pH
Indicators are chemical dyes used to determine pH of substances.
They are cheap, but only precise at specific temperatures and have limited range
A pH meter is a device that uses two electrodes to measure pH.

14. PRACTICE PROBLEM #1
What is the pH of a solution where [H+] = 1.0x10-3 M?

15. PRACTICE PROBLEM #2
What is the hydrogen ion concentration of a solution with a pH of 9.6?

16. PRACTICE PROBLEM #3
What is the pOH of a solution with a pH of 6.2?

17. PRACTICE PROBLEM #4
What is the pOH of a solution with a [H+] = 2.9x10-11M?

18. PRACTICE PROBLEM #5
Find [OH-], pH and pOH of a solution where [H+]=7.2x10-2M.