1. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 1
Chapter 10 Acids and Bases
10.4
The pH Scale

2. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 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. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 3
pH of Everyday Substances

4. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 4
Identify each solution as A) acidic, B) basic, or
N) neutral
___ 1) HCl with a pH = 1.5
___ 2) pancreatic fluid [H3O+
] = 1 x 10−8
M
___ 3) Sprite soft drink, pH = 3.0
___ 4) pH = 7.0
___ 5) [OH−
] = 3 x 10−10
M
___ 6) [H3O+
] = 5 x 10−12
M
Learning Check

5. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 5
Identify each solution as A) acidic, B) basic, or
N) neutral
A 1) HCl with a pH = 1.5
B 2) Pancreatic fluid [H3O+
] = 1 x 10−8
M
A 3) Sprite soft drink pH = 3.0
N 4) pH = 7.0
A 5) [OH-
] = 3 x 10−10
M
B 6) [H3O+
] = 5 x 10−12
Solution

6. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 6
Testing the pH of Solutions
The pH of solutions can be determined using
 a pH meter
 pH paper
 indicators that have specific colors at different pH
values

7. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 7
 Mathematically, pH is the negative log of the
hydronium ion concentration
pH = −log [H3O+
]
 For a solution with [H3O+
] = 1 x 10−4
,
pH = −log [1 x 10−4
]
pH = −[−4.0]
pH = 4.0
Calculating pH

8. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 8
Significant Figures in pH
When expressing log values, the number of decimal
places in the pH is equal to the number of significant
figures in the coefficient of [H3O+
].
coefficient decimal places
[H3O+
] = 1 x 10−4
pH = 4.0
[H3O+
] = 8.0 x 10−6
pH = 5.10
[H3O+
] = 2.4 x 10−8
pH = 7.62

9. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 9
Guide to Calculating pH

10. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 10
Find the pH of a solution with a [H3O+
] of 1.0 x 10−3
.
STEP 1 Enter the [H3O+
] value:
Enter 1 x 10−3
(press 1 EE 3, then change sign)
The EE key gives the exponent of 10.
STEP 2 Press log key and change the sign:
−log (1 x 10−3
) = −[−3]
STEP 3 Make the number of digits after the decimal
point (2) equal to the number of significant figures in
the coefficient (2):
[H3O+
] = 1.0 x 10−3
pH is 3.00
Example of Calculating pH

11. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 11
Learning Check
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

12. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 12
Solution
What is the pH of coffee if the [H3O+
] is 1 x 10−5
M? STEP
1 Enter the [H3O+
] value:
Enter 1 x 10−5
(press 1 EE 5, then change
sign )
STEP 2 Press log key and change the sign:
−log (1 x 10−5
) = −[−5]
STEP 3 Make the number of digits after the decimal
point (1) equal to the number of significant
figures in the coefficient (1):
[H3O+
]= 1 x 10−5
, pH is 5.0 (3)

13. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 13
The [H3O+
] of tomato juice is 2 x 10−4
M. What is the
pH of the solution?
1) 4.0
2) 3.7
3) 10.3
Learning Check

14. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 14
The [H3O+
] of tomato juice is 2 x 10−4
M. What is the pH of
the solution?
STEP 1 Enter the [H3O+
] value:
Enter 2 x 10−4
(press 2 EE 4, then change
sign )
STEP 2 Press log key and change the sign:
−log (2 x 10−4
) = −[−3.7]
STEP 3 Make the number of digits after the decimal
point (1) equal to the number of significant
figures in the coefficient (1):
[H3O+
] = 2 x 10−4
, pH is = 3.7 (2)
Solution

15. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 15
The [OH−
] of a solution is 1.0 x 10−3
M. What is the pH?
1) 3.00
2) 11.00
3) –11.00
Learning Check

16. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 16
The [OH−
] of a solution is 1.0 x 10−3
M. What is the pH?
STEP 1 Enter the [H3O+
] value:
Use the Kw to obtain [H3O+
] = 1.0 x 10−11
M
Enter 1.0 x 10−11
(press 1 EE 11, then change
sign)
STEP 2 Press log key and change the sign:
−log (1.0 x 10−11
) = −[−11]
STEP 3 Make the number of digits after the decimal
point (2) equal to the number of significant figures in
the coefficient (2):
[H3O+
] =1.0 x 10−11
, pH is = 11.00 (2)
Solution

17. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 17
[H3O+
], [OH-
], and pH Values

18. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 18
Example 1: Calculating [H3O+
]
from pH
Calculate the [H3O+
] for a pH value of 8.0.
[H3O+
] = 1 x 10−pH
For pH = 8.0, the [H3O+
] = 1 x 10−8
STEP 1 Enter the pH value, change sign: –8.0
STEP 2 Convert −pH to concentration:
Use 2nd
function key and then10x
key
or inverse key and then log key
1 −08
STEP 3 Adjust the significant figures in the coefficient (1
digit following decimal point = 1 digit in the coefficient):
[H3O+
] = 1 x 10−8
M

19. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 19
Example 2: Calculating [H3O+
]
from pH
Calculate the [H3O+
] for a pH of 3.80.
STEP 1 Enter the pH value, change sign: –3.80
STEP 2 Convert −pH to concentration:
Use 2nd
function key and then10x
key
or inverse key and then log key
1.584893 −06
STEP 3 Adjust the significant figures in the coefficient
(2 digit following decimal point = 2 digit in the
coefficient):
[H3O+
] = 1.6 x 10−6
M

20. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 20
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
Learning Check

21. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 21
What is the [H3O+
] of a solution with a pH of 10.0?
STEP 1 Enter the pH value, change sign: –10.0
STEP 2 Convert −pH to concentration:
Use 2nd
function key and then10x
key
or inverse key and then log key
1−10
STEP 3 Adjust the significant figures in the coefficient
(1 digit following decimal point = 1 digit in the
coefficient):
[H3O+
] = 1 x 10−10
M (3)
Solution

22. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 22
What is the [H3O+
] of a solution with a pH of 2.85?
1) 1.0 x 10−2.85
M
2) 1.4 x 10−3
M
3) 8.5 x 10−2
M
Learning Check

23. General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 23
What is the [H3O+
] of a solution with a pH of 2.85?
STEP 1 Enter the pH value, change sign: –2.85
STEP 2 Convert −pH to concentration:
Use 2nd
function key and then10x
key
or inverse key and then log key
0.0014125 = 1.4125 x 10−03
STEP 3 Adjust the significant figures in the coefficient (2
digits following decimal point = 2 digits in the
coefficient):
[H3O+
] = 1.4 x 10−3
M (2)
Solution