1. Chapter 14 Acids and Bases
14.6 The pH Scale
Learning Goal Calculate the pH from [H3O+]; given the pH, calculate the [H3O+] and [OH−] of a solution.
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2. is used to indicate the acidity of a solution
The 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
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3. pH of Common Substances
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4. Identify each solution as acidic, basic, or neutral.
Learning Check
Identify each solution as acidic, basic, or neutral.
A. HCl with a pH = 1.5
pancreatic fluid [H3O+] = 1 × 10−8 M
C. soft drink pH = 3.0
D. pH = 7.0
[OH−] = 3 × 10−10 M
[H3O+ ] = 5 × 10−12
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5. Identify each solution as acidic, basic, or neutral.
A. HCl with a pH = acidic
pancreatic fluid [H3O+] = 1 × 10−8 M basic
C. soft drink pH = acidic
D. pH = neutral
[OH−] = 3 × 10−10 M acidic
[H3O+ ] = 5 × 10− basic
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6. The pH of solutions is determined by using a pH meter pH paper, or
Testing for pH
The pH of solutions is determined by using
a pH meter
pH paper, or
indicators that have specific colors at different pH values
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7. For a solution with [H3O+] = 1.0 × 10−4 M pH = −log [1.0 × 104 ]
Calculating pH
pH is defined as the negative logarithm (log) of the hydronium ion concentration.
pH = −log[H3O+]
For a solution with [H3O+] = 1.0 × 10−4 M
pH = −log [1.0 × 104 ]
pH = −[−4.00]
pH = 4.00
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8. pH, Significant Figures
When expressing pH values, the number of
decimal places is equal to the number of
significant figures in the coefficient of [H3O+].
coefficient decimal places
[H3O+] = 1 × 104 pH = 4.0
[H3O+] = 2.4 × 108 pH = 7.62
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9. Guide to Calculating pH
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10. Learning Check
Acetylsalicylic acid, known as aspirin, was the first nonsteroidal anti-inflammatory drug used to alleviate pain and fever. If a solution of aspirin has a [H3O+] = 1.7 × 10−3 M, what is the pH of the solution?
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11. Step 1 State the given and needed quantities.
Solution
If a solution of aspirin has a [H3O+] = 1.7 × 10−3 M, what is the pH of the solution?
Step 1 State the given and needed quantities.
Given
Need
Know
[H3O+] = 1.7 × 10−3 M pH
pH = −log[H3O+]
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12. Solution
If a solution of aspirin has a [H3O+] = 1.7 × 10−3 M, what is the pH of the solution? Step 2 Enter the [H3O+] into the pH equation. pH = −log[H3O+] = −log[1.7 × 10−3 ] Procedure: Enter 1.7, press Enter 3, press to change sign.
EE or Exp
/−+
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13. Solution
If a solution of aspirin has a [H3O+] = 1.7 × 10−3 M, what is the pH of the solution? Step 3 Press the log key and change the sign. Procedure: Press Calculator display Adjust the number of SFs on the right of the decimal point. pH = 2.77
log
+/−
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14. Calculate the pH of a solution with [OH−] of 4.0 × 10−5 M.
Learning Check
Calculate the pH of a solution with [OH−] of 4.0 × 10−5 M.
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15. Calculate the pH of a solution with [OH−] of 4.0 × 10−5 M.
Step 1 State the given and needed quantities.
Given
Need
Know
[OH−] = 4.0 × 10−5 M
[H3O+] pH
Kw = [H3O+] [OH−]
pH = −log[H3O+]
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16. Calculate the pH of a solution with [OH−] of 4.0 × 10−5 M.
Step 2 Enter the [H3O+] into the pH equation.
pH = −log[2.5 × 10−10] =
EE or Exp
+/− =
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17. Solution
Calculate the pH of a solution with [OH−] of 4.0 × 10−5 M. Step 3 Press the log key and change the sign. Press Calculator display Adjust the number of SFs on the right of the decimal point. pH = 9.60
log
+/−
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18. a high [OH−] have low pOH values a low [OH−] have high pOH values
The pOH scale is similar to the pH scale, except that pOH is associated with the OH− concentration of an aqueous solution.
pH = −log[OH−]
Solutions with
a high [OH−] have low pOH values
a low [OH−] have high pOH values
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19. In any aqueous solution, pH + pOH = 14.00 and pOH = 14 − pH
In a solution with a pH of 3.50, we can calculate the pOH from the pH.
pOH = 14 − 3.50 = 10.50
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20. pH and pOH
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21. pH and pOH
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22. Calculating [H3O+] from pH
Given the pH of a solution, [H3O+] can be calculated:
[H3O+] = 10−pH
If the pH of a solution is 3.0:
[H3O+] = 1 × 10−3 M
For pH values that are not whole numbers, the 10x key on your calculator can be used.
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23. Guide to Calculating [H3O+] from pH
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24. Calculate [H3O+] for a solution of baking soda with a pH of 8.25.
Learning Check
Calculate [H3O+] for a solution of baking soda with a pH of 8.25.
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25. Calculate [H3O+] for a solution of baking soda with a pH of 8.25.
Step 1 State the given and needed quantities.
Given
Need
Know
pH = 8.25
[H3O+]
[H3O+] = 10−pH
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26. Calculate [H3O+] for a solution of baking soda with a pH of 8.25.
Step 2 Enter the pH value into the inverse log equation and change the sign.
= −8.25
Press the 2nd function key and then the 10x key.
= −09
+/−
2nd
10x
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27. Solution
Calculate [H3O+] for a solution of baking soda with a pH of Step 3 Adjust the SFs in the coefficient. [H3O+] = 5.6 × 10−9 M
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