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1
Chapter 14 Acids and Bases
14.6
The pH Scale
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
2
pH Scale
The pH scale:
• 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
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
4
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
5
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 • pH meter, • pH paper, or • indicators that have specific colors at different
pH values.
Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings
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pH RangeTable 14.8
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
8
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
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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+].
[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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Guide to pH Calculations
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
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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 significant figures after decimal point
3 3.00 Two significant figures in 1.0 x 10−3
Calculating pH
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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) 3.7 pH = - log [ 2 x 10-4] = 3.7
2 (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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pOH
The pOH of a solution • Is analogous to the pH value on the pH scale.• Is based on the [OH−].• Has high pOH values with low [OH−] and high
[H3O+].
• Is equal to the - log [OH-] pOH = - log [OH-].• Added to the pH value is equal to 14.0.
pH + pOH = 14.0
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Calculation pOH
Find the pOH of a solution with a [OH−] of 2.0 x 10−3.
STEP 1 Enter [OH−]
STEP 2 Press log key and change sign
- log (2.0 x 10−3) = -[-2.69897] = 2.69897
STEP 3 Adjust figures after decimal point
2.70 Two significant figures in 2.0 x 10−3
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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
If the pH is 3.0, the [H3O+] = 1 x 10−3
On a calculator
1. Enter the pH value 3.0
2. Change sign -3.0
3. 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 [H3O+] of a solution with a pH of 5.75?
1) 1.8 x 10− 6 M 2) 1.778 x 10−6 M
3) 2 x 10−6 M
Learning Check
18
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 [H3O+] of a solution with a pH of 5.75?
1) 1.8 x 10− 6 M5.75 (+/-) inv log (or 10x) = 1.8 x 10-6
Note: Two decimal places (.75), indicate two significant figures in the coefficient of the answer.
Solution
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Learning Check
What is the pH and the pOH of coffee if the [H3O+] is 1 x 10−5 M?
1) pH = 5.0 pOH =7.0
2) pH = 7.0 pOH = 9.0
3) pH = 5.0 pOH = 9.0
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Solution
What is the pH and the pOH of coffee if the [H3O+] is 1 x 10−5M?
3) pH = 5.0 pOH = 9.0
pH = -log [1 x 10−5] = -(-5.0) = 5.0
pH + pOH = 14.0
pOH = 14.0 – pH = 14.0 – 5.0 = 9.0
or [OH-] = 1 x 10−9
pOH = - log [1 x 10−9] = -(-9.0) = 9.0