pH/pOH Calculations
Chemistry
5/28/15
Drill



What is the formula to calculate pH?
What is the range of the pH scale?
Classify these as acid, base, or neutral:
•
•
•
•

pH = 13.7
pH = 2.3
pH = 7.0
pH = -1.1
HW: Acids and Bases Review #29-34
Objectives
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IWBAT
• Calculate pH, pOH, [H+], [OH-] for any
aqueous solution, given one of the other
values.
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NOTES:
• Titration Lab–6/2 & 6/3—Tuesday and
Wednesday – CLOSED TOE SHOES
• Acids & Bases Test – Friday 6/5
HW

Hold onto 18-1 Review &
Reinforcement—we’ll go over it next
week.
pH Calculations

pH is based on the
concentration of H+ ion in a
solution. The formula is:
• pH = -log[H+]

Or it may be rearranged to:
• [H+] = 10-pH (10 to the power of
-pH)
• So, since pure water has pH =
7:
• [H+]H2O = 1.0 x 10 -7 M
Acid------Neutral-----Base
[H+]
[OH-]
0---------------7------------14
Example Calculations

The concentration of
hydrogen ions in a
solution is 5.21 x 10-2
M. What is its pH?
• pH = -log[H+]
• = -log(5.21 x 10-2 M)
• = 1.28

Calculate [H+] for a
solution with a pH of
8.32.
• [H+] = 10-pH
• = 10-8.32
• = 4.8 x 10-9 M
What about pOH?
And the rest?


Look at the back of 19-1 Practice
Problems—let’s take a look at this chart
NOTE: [H+] = [H3O+]
pH + pOH = 14
pH
pOH
pOH = -log[OH-]
[OH-] = 10-pOH
[H+] = 10-pH
pH = -log [H+]
pH + pOH = 14
[H+][OH-] = 1.0 x 10-14
[OH-]
[H+]
[H+][OH-] = 1.0 x 10-14
Let’s use it!

Take out pH and pOH WS (got on
Monday)
pH and pOH
1
2
3
4
5
6
7
8
9
10
[H+]
10-5 M
pH
5
7
[OH-]
10-9 M
pOH
9
10-4 M
10-2 M
11
12
10-5 M
10-11 M
13
6
A/B
acidic
Let’s make it more
challenging!


Turn to 19-1 Practice Problems (front of
the chart)
To determine acid, base, or neutral,
what do you need to know?
19-1 Prac Probs
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
[OH-] = 2.51 x 10-2 M, base, pH = 12.4
[H3O+] = 1.0 x 10-6 M, acid, pH = 6
[OH-] = 6.2 x 10-9 M, acid, pH = 5.8
[H3O+] = 7.94 x 10-12 M, base, pH = 11.1
[OH-] = 1.7 x 10-8 M, acid, pH = 6.2
[H3O+] = 3.16 x 10-4 M, acid, pH = 3.50
[OH-] = 1.58 x 10-5 M, base, pH = 9.20
[H3O+] = 1.7 x 10-8 M, base, pH = 7.8
[OH-] = 2.51 x 10-6 M, base, pH = 8.40
[H3O+] = 3.16 x 10-7 M, acid, pH = 6.50
Closure
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Drano is mainly sodium hydroxide. The
[OH-] is often around 0.35 M.
Calculate the pH, pOH, and [H+]