KEY
HOMEWORK for Chapter 14: Acids and Bases
14.6 The pH Scale
(Read pgs. 458-464 in the chemistry textbook)
1. Locate and label each of the following on the pH scale below:
a) neutral pH
b) use an arrow to show the 'range' of increasingly acidic pH values
c) use an arrow to show the 'range' of increasingly basic pH values
d) draw a box around the pH that is 100X more basic than pH 10
e) draw a triangle around the the pH that is 1,000X more acidic than pH 9
f) underline the pH of hydrochloric acid
g) draw a circle around the pH of 1.0M NaOH (lye)
The pH Scale
neutral pH
1
2
3
4
5
6
7
increasingly acidic pH
8
9
10
11
12
13
14
increasingly basic pH
2. Fill in the following table to the relationship of pH values to [H+]
Solution
Acidic
[H+]
pH
> 1.0 x 10-7 M
= 1.0 x 10-7 M
< 1.0 x 10-7 M
< 7.0
= 7.0
> 7.0
Neural
Basic
3. Identify the regions of the pH scale where:
a) [H+] is increasing relative to neutral
b) H+] is decreasing relative to neutral
c) [OH-] is increasing relative to neutral
d) [OH-]is decreasing relative to neutral
The pH Scale
1
2
3
4
5
6
7
8
9
10
11
increasing [H+]
decreasing [H+]
decreasing [OH-]
increasing [OH-]
12
13
14
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4. Consider the pH of the following items:
Item
root beer
kitchen cleaner
pickles
glass cleaner
cranberry juice
pH
5.8
10.9
3.5
7.6
2.9
a) Place the pH values of the items on the list in order of most acidic to most basic.
Most
Most
Acidic
Basic
cranberry
juice
> pickles
> rootbeer
> glass > kitchen
cleaner cleaner
b) Which item has the highest [H+]?
cranberry juice
c) Which item has the highest [OH-]?
kitchen cleaner
5. If the pH of a solution reads 4.00, is the solution
acidic / basic / neutral ?
(circle one)
6. What does it mean when we say that the pH scale is a logarithmic scale?
each number on the scale represents an exponent, called a
logarithm, to which the base 10 must be raised to equal the [H+]
of a solution relative to that of neutral water. What we really
mean is how many times we need to multipy 10 by itself to equal
a given number....
....except that neutral water has an [H+] of 1.0 x 10-7 𝒎𝒐𝒍𝒆𝒔
....a
𝒍𝒊𝒕𝒆𝒓
number less than 10, so we must raise base 10 to the negative
value of the pH.....the negative log is usually called the antilogarithim, or antilog for short
neutral pH = 7.00 ....so.... 10-7.00 = 1.0 x 10-7M = [H+]
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6. Continued:
[H+] is the antilog of the pH
[H+] = 10-pH
Example:
pH
7.00
6.00
5.00
4.00
7.
antilog
10-7.00
10-6.00
10-5.00
10-4.00
a) what is the [H+] of a solution with a pH of 2.00?
Is this solution acidic or basic?
Is this solution acidic or basic?
Is this solution acidic or basic?
Is this solution acidic or basic?
= BASIC
10-12.0 =1 x 10-12 M
1 x 10-12 M << 1 x 10-7 M,
e) what is the [H+] of a solution with a pH of 6.20?
= ACIDIC
10-8.30 = 5.0 x 10-9 M
5.0 x 10-9 M < 1 x 10-7 M,
d) what is the [H+] of a solution with a pH of 12.0?
= ACIDIC
10-2.5 = 0.003 M
0.003M >> 1 x 10-7 M,
c) what is the [H+] of a solution with a pH of 8.30?
M
M
M
M
10-2.00 = 0.01 M
0.01 M >> 1 x 10-7 M,
b) what is the [H+] of a solution with a pH of 2.50?
Is this solution acidic or basic?
[H+]_________
= 0.000 000 100
= 0.000 00 100
= 0.000 0100
= 0.000 100
= BASIC
10-6.20 =6.310 x 10-7 M
0.010 M >> 1 x 10-7 M,
= BASIC
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8. Conversely, if you already know the [H+] value, you can find out the pH by taking the log of the [H+].
This answer will be ( − ), so we multiply the answer by −1 to make the answer ( + )
pH is the negative log (base 10) of the [H+] concentration
pH = - log [H+]
Example:
_______H+]
− log[0.000 000 100]
− log [0.000 00 100]
− log [0.000 0100]
− log [0.000 100]
___pH
= 7.00
= 6.00
= 5.00
= 4.00
-log(1 x 10-6) = 6.0
a) what is the pH of a solution with an [H+] of 1 x 10-6 M?
Is this solution acidic or basic?
pH 6.0 < pH 7
b) what is the pH of a solution with an [H+] of 1 x 10-10 M?
Is this solution acidic or basic?
-log(1 x 10-10) = 10.0
pH 10.0 > pH 7 = BASIC
c) what is the pH of a solution with an [H+] of 1.5 x 10-1 M?
Is this solution acidic or basic?
-log(1.5 x 10-1) = 0.82
pH 6.0 << pH 7 = ACIDIC
d) what is the pH of a solution with an [H+] of 3.66 x 10-5 M?
Is this solution acidic or basic?
-log(3.66 x 10-5) = 4.437
pH 4.437 < pH 7 = ACIDIC
e) what is the pH of a solution with an [H+] of 5 x 10-8 M?
Is this solution acidic or basic?
= ACIDIC
-log(5.0 x 10-15) = 14
pH 14 >> pH 7 = BASIC
4
9. If you know the [H+] it is easy to find the [OH-] concentration by remembering that KW = [H+] [OH-] and
that the value of KW = 1.0 x 10-14 M
-
[OH ] =
𝑲𝑾
[𝑯+ ]
(a) What is the [OH-] of a solution with an [H+] of 3.7 x 10-3?
[OH-] =
𝟏.𝟎 𝒙 𝟏𝟎−𝟏𝟒
𝟑.𝟕 𝒙 𝟏𝟎−𝟑
→ 2.7 x 10-12 M
(b) What is the [OH-] of a solution with an [H+] of 5.5 x 10-13?
-
[OH ] =
𝟏.𝟎 𝒙 𝟏𝟎−𝟏𝟒
𝟓.𝟓 𝒙 𝟏𝟎−𝟏𝟑
→ 0.18 M
(c) What is the [OH-] of a solution with an [H+] of 2.5 x 10-9?
-
[OH ] =
𝟏.𝟎 𝒙 𝟏𝟎−𝟏𝟒
𝟐.𝟓 𝒙 𝟏𝟎−𝟗
→ 4.0 x 10-6 M
10. Conversely, if you already know the [H+] you can use the KW to calculate the [OH-] value.
+
[H ] =
𝑲𝑾
[𝑶𝑯− ]
(a) What is the [H+] of a solution with an [OH-] of 2 x 10-10?
+
[H ] =
𝟏.𝟎 𝒙 𝟏𝟎−𝟏𝟒
𝟐 𝒙 𝟏𝟎−𝟏𝟎
→ 5 x 10-5 M
(b) What is the [H+] of a solution with an [OH-] of 3.8 x 10-4?
[H+] =
𝟏.𝟎 𝒙 𝟏𝟎−𝟏𝟒
𝟑.𝟖 𝒙 𝟏𝟎−𝟒
→ 2.6 x 10-11 M
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12. What is the pOH
scale?
a scale similar to the pH scale except that pOH is associated with
[OH-] of an aqueous solution
13. The antilog of the [OH-] will give the pOH value.
pOH is the negative log (base 10) of the OH-] concentration
pOH = - log [OH-]
a) what is the pOH of a solution with an [OH-] of 1 x 10-6 M?
-log(1 x 10-6) = 6.0
b) what is the pOH of a solution with an [OH-] of 1 x 10-10 M?
-log(1 x 10-10) = 10.0
c) what is the pOH of a solution with an [OH-] of 1.5 x 10-1 M?
-log(1.5 x 10-1) = 0.82
14. The sum of the pH and the pOH will always equal 14 - so if you know either value you can quickly
calculate the other value by subtracting:
pOH + pH = 14.00
(a) What is the pH of a solution with a pOH of 10.0?
14.00 = 10.0 + x → pH = 4.0
(a) What is the pOH of a solution with a pH of 5.0?
14.00 = x + 5.0 →
pOH = 9.0
(a) What is the pH of a solution with a pOH of 7.5?
14.00 = 7.5 + x →
pH = 6.5
15. Why does a neutral solution have a pH of 7.00?
b/c the [H+] = [OH-] so they cancel each other out
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16. If you know the [OH-], how can you determine the pH of solution?
STEP 1: take - log [OH-]
STEP 2: subtract answer from 14.00
17. Calculate the pH of each solution given the following [H+] or [OH-] values:
(a) [OH-] = 1.0 x 10-5 M
STEP 1: - log [1.0 x 10-5] = 5.00
STEP 2: 14.00 - 5.00 = 9.00
(b) [OH-] = 2.5 x 10-11
STEP 1: - log [2.5 x 10-11] = 10.60
STEP 2: 14.00 - 10.60 = 3.40
(c) [H+] = 1.0 x 10-4
STEP 1: - log [1.0 x 10-4] = 4.00
(d) [OH-] = 8.2 x 10-4
STEP 1: - log [8.2 x 10-4] = 3.08
STEP 2: 14.00 - 3.08 = 10.92
(e) [H+] = 3.6 x 10-9
STEP 1: - log [3.6 x 10-9] = 8.44
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18. A solution with a pH of 3 is 10 times more acidic than a solution with a pH of 4. Explain.
An increase or decrease of one pH unit changes the [H+] by a
factor of 10. Thus a pH of 3 is 10 times more acidic than a pH of
19. State whether each of the following solutions is acidic, basic, or neutral:
ACIDIC
(a) soda, pH 3.22
BASIC
(b) laundry detergent, pOH 4.56 → 14.00 - 4.56 = 9.44 pH
ACIDIC
(c) honey, pH 3.9
ACIDIC (d)
shampoo, pOH 8.3 → 14.00 - 8.3 = 5.70 pH
ACIDIC (e)
rain, pH 5.8
BASIC
(f) cheese, pH 7.4
ACIDIC (g)
coffee, pH 5.52
BASIC
(h) drain cleaner, pOH 2.8 → 14.00 - 2.8 = 11.20 pH
ACIDIC
(i) tomatoes, pH 4.2
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