1. Chapter 8 Acids and Bases
8.5
The pH Scale
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Publishing as Benjamin Cummings

2. pH Scale The pH of a solution
is used to indicate the acidity of a solution.
has values that usually range from 0 to 14.
is acidic when the values are less than 7.
is neutral with a pH of 7.
is basic when the values are greater than 7.

3. pH of Everyday Substances
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

4. Learning Check Identify each solution as 1) acidic 2) basic 3) neutral
A. ___ HCl with a pH = 1.5
B. ___ pancreatic fluid [H3O+] = 1 x 10−8 M
C. ___ Sprite® soft drink pH = 3.0
D. ___ pH = 7.0
E. ___ [OH−] = 3 x 10−10 M
F. ___ [H3O+ ] = 5 x 10−12

5. Solution A. 1 HCl with a pH = 1.5
B Pancreatic fluid [H3O+] = 1 x 10−8 M
C Sprite® soft drink pH = 3.0
D. 3 pH = 7.0
E. 1 [OH-] = 3 x 10−10 M
F [H3O+] = 5 x 10−12

6. Testing the pH of Solutions
The pH of solutions can be determined using
a) pH meter
b) pH paper
c) indicators that have specific colors at different pH values.

7. Calculating pH
pH is the negative log of the hydronium ion concentration.
pH = - log [H3O+]
Example: For a solution with [H3O+] = 1 x 10−4 M
pH = −log [1 x 10−4 M]
pH = - [-4.0]
pH = 4.0
Note: The number of decimal places in the pH equals
the significant figures in the coefficient of [H3O+].
4.0
1 SF in 1 x 10-4 M

8. Calculating pOH
pOH is the negative log of the hydroxide ion concentration.
pOH = - log [OH−]
Example: For a solution with [OH−] = 1 x 10−12 M
pH = −log [1 x 10−12 M]
pH = - [-12.0]
pH = 12.0
Note: The number of decimal places in the pOH equals
the significant figures in the coefficient of [OH−].
SF in 1 x M
Note: pKw = 14 = pH + pOH

9. Guide to pH or pOH Calculations

10. Significant Figures in pH and pOH
When expressing log values, the number of decimal
places in the pH or pOH is equal to the number of significant
figures in the coefficient of [H3O+] or [OH−].
[H3O+] = 1 x 10-4 pH = 4.0
[H3O+] = 8.0 x 10-6 pH = 5.10
[OH−] = 1 x pOH = 12.0
[OH−] = 2.4 x 10-8 pOH = 7.62

11. Calculating pH Find the pH of a solution with a [H3O+] of 1.0 x 10−3:
STEP 1 Enter [H3O+]
Enter 1 x by pressing 1 (EE) 3
The EE key gives an exponent of 10 and
change sign (+/- key or – key)
STEP 2 Press log key and change sign
- log (1 x 10−3) = -[-3]
STEP 3 Adjust figures after decimal point to equal the
significant figures in the coefficient.
Two significant figures in 1.0 x 10−3

12. Learning Check 1) pH = 9.0 2) pH = 7.0 3) pH = 5.0
What is the pH of coffee if the [H3O+] is 1 x 10−5 M?
1) pH = 9.0
2) pH = 7.0
3) pH = 5.0

13. Solution 3) pH = 5.0 pH = -log [1 x 10−5] = -(-5.0) = 5.0
What is the pH of coffee if the [H3O+] is 1 x 10−5M?
3) pH = 5.0
pH = -log [1 x 10−5] = -(-5.0) = 5.0

14. Learning Check A. The [H3O+] of tomato juice is 2 x 10−4 M.
What is the pH of the solution?
1) ) )
B. The [OH−] of a solution is 1.0 x 10−3 M.
1) ) )

15. Solution A. 2) 3.7 pH = - log [ 2 x 10-4] = 3.7
2 (EE) 4 (+/-) log (+/-)
B. 2)
Use the Kw to obtain [H3O+] = =1.0 x 10 −11
pH = - log [1.0 x 10 −11]
1.0 (EE) 11 (+/-) log (+/-)
[ 1 x 10-14]
[ 1 x 10-3]

16. [H3O+], [OH-], and pH Values
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

17. Calculating [H3O+] from pH
The [H3O+] can be expressed by using the pH as the negative power of 10.
[H3O+] = 1 x 10 -pH
For pH = 3.0, the [H3O+] = 1 x M
On a calculator
1. Enter the pH value
2. Change sign
3. Use the inverse log key (or 10x) to obtain
the [H30+]. = 1 x M

18. Calculating [OH−] from pOH
The [OH−] can be expressed by using the pOH as the negative power of 10.
[OH−] = 1 x 10 -pOH
For pOH = 10.0, the [OH−] = 1 x M
On a calculator
1. Enter the pOH value
2. Change sign
3. Use the inverse log key (or 10x) to obtain
the [OH−] = 1 x M

19. Learning Check A. What is the [H3O+] of a solution with a pH of 10.0?
1) 1 x 10−4 M
2) 1 x 1010 M
3) 1 x 10−10 M
B. What is the [OH−] of a solution with a pH of 2.00?
1) 1.0 x 10− 2 M
2) 1.0 x 10−12 M
3) M

20. Solution A. What is the [H3O+] of a solution with a pH of 10.0?
3) 1 x M 1 x 10-pH
B. What is the [OH−] of a solution with a pH of 2.00?
2) 1.0 x 10−12 M
[H3O+] = 1.0 x 10−2 M 1 x 10-pH
[OH−] = 1.0 x 10−14 M2 = 1.0 x 10−12 M, use Kw!
1.0 x 10−2 M