Download - 10.4 the p_h_scale
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General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 1
Chapter 10 Acids and Bases
10.4The pH Scale
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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
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General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 3
pH of Everyday Substances
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Identify each solution as A) acidic, B) basic, orN) 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
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Identify each solution as A) acidic, B) basic, orN) 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
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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
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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
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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+].
coefficient decimal places
[H3O+] = 1 x 10−4 pH = 4.0
[H3O+] = 8.0 x 10−6pH = 5.10
[H3O+] = 2.4 x 10−8pH = 7.62
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General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 9
Guide to Calculating pH
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Find the pH of a solution with a [H3O+] of 1.0 x 10−3.
STEP 1 Enter the [H3O+] value:
Enter 1 x 103 (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
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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 10−5 M? STEP 1 Enter the [H3O+] value: Enter 1 x 105 (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)
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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
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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 104 (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
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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
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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 MEnter 1.0 x 10−11 (press 1 EE 11, then changesign)
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
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General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 17
[H3O+], [OH-], and pH Values
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Example 1: Calculating [H3O+] from pHCalculate 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
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Example 2: Calculating [H3O+] from pHCalculate 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
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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
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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
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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
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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