1. Chapter 20 Notes, part III
pH and [H+]

2. Hydrogen and hydroxide in H2O
Working with the Arrhenius acid definition, we say that acids are:
HX a H+ + X-
And that a base would be:
XOH a X+ and OH-

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

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

5. Hydrogen and hydroxide in H2O
Kw=[H+]x[OH-]
Kw=(1.0x10-7M)x(1.0x10-7M)
Kw= 1x10-14

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-.

7. The pH Scale
The pH scale is a logarithmic scale from 0 to 14. It measures the concentration of H+ (pH stands for “potential hydrogen”) with the formula:
pH = -log[H+]

8. The pH Scale Water has a pH of 7.0, which is neutral.
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).

9. The pH Scale
Another way to look at it is the concentration of hydrogen in a solution is 1x10-xM. X is the pH.

10. The pOH Scale The pOH scale measures the hydroxide ion concentration.
Because Kw=[H+][OH-]=1x10-14, 14=pH + pOH
Given this, you can solve for pH, pOH and [OH-] given just [H+].

11. How to measure pH
Indicators-chemical dyes used to determine pH of substances
Cheap, but only precise at specific temperatures and have limited range
pH meter-device that uses two electrodes to measure pH

12. Practice Problem #1
What is the pH of a solution where [H+] = 1.0x10-3M?

13. Practice Problem #2
What is the hydrogen ion concentration of a solution with a pH of 9.6?

14. Practice Problem #3
What is the pOH of a solution with a pH of 6.2?

15. Practice Problem #4
What is the pOH of a solution with a [H+]=2.9x10-11M?

16. Practice Problem #5
Find [OH-], pH and pOH of a solution where [H+]=7.2x10-2M.