Post on 31-Mar-2015
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Chapter 8 Acids and Bases
8.4 The pH Scale
Copyright © 2009 by Pearson Education, Inc.
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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.
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pH of Everyday Substances
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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
Learning Check
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A. 1 HCl with a pH = 1.5
B. 2 pancreatic fluid [H3O+] = 1 x 10−8 M
C. 1 Sprite® soft drink pH = 3.0
D. 3 pH = 7.0
E. 1 [OH−] = 3 x 10−10 M
F. 2 [H3O+] = 5 x 10−12
Solution
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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.
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pH is the negative log of the hydronium ion concentration.pH = - log [H3O+]
Example: For a solution with [H3O+] = 1 x 10−4
pH = −log [1 x 10−4 ]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
Calculating pH
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Guide to Calculating pH of an Aqueous Solution
Copyright © 2009 by Pearson Education, Inc.
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Significant Figures in pH
When expressing log values, the number of decimalplaces in the pH is equal to the number of significant figures in the coefficient of [H3O+].
[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
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Find the pH of a solution with a [H3O+] of 1.0 x 10−3: STEP 1: Enter [H3O+]
Enter 1 x 10-3 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.
3 3.00 Two significant figures in 1.0 x 10−3
Calculating pH
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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
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Solution
What is the pH of coffee if the [H3O+] is 1 x 10−5M?
3) pH = 5.0pH = -log [1 x 10−5] = -(-5.0) = 5.0
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A. 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
B. The [OH−] of a solution is 1.0 x 10−3 M. What is the pH of the solution?
1) 3.00 2) 11.00 3) -11.00
Learning Check
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A. 2) 3.7pH = - log [ 2 x 10-4] = 3.72 (EE) 4 (+/-) log (+/-)
B. 2) 11.00 Use the Kw to obtain [H3O+] = 1.0 x 10 −11
pH = - log [1.0 x 10 −11] 1.0 (EE) 11 (+/-) log (+/-)
Solution
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[H3O+], [OH−], and pH Values
Copyright © 2009 by Pearson Education, Inc.
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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 10 -3
On a calculator1. Enter the pH value 3.02. Change sign -3.03. Use the inverse log key (or 10x) to obtain
the [H30+]. = 1 x 10 -3 M
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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) 2.0 M
Learning Check
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A. What is the [H3O+] of a solution with a pH of 10.0?
3) 1 x 10-10 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 = 1.0 x 10−12 M 1.0 x 10−2
Solution