Chapter 14 Acids and Bases

Basic Chemistry Copyright © 2011 Pearson Education, Inc.1

Chapter 14 Acids and Bases

14.6

The pH Scale


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


pH of Everyday Substances


Identify each solution as

A) acidic, B) basic, or N) neutral.

1. ___ HCl with a pH = 1.5

2. ___ pancreatic fluid [H3O+] = 1 x 10−8 M

3. ___ soft drink pH = 3.0

4. ___ pH = 7.0

5. ___ [OH−] = 3 x 10−10 M

6. ___ [H3O+ ] = 5 x 10−12

Learning Check


1. A) HCl with a pH = 1.5

2. B) pancreatic fluid [H3O+] = 1 x 10−8 M

3. A) soft drink pH = 3.0

4. N) pH = 7.0

5. A) [OH-] = 3 x 10−10 M

6. B) [H3O+] = 5 x 10−12 M

Solution


Testing the pH of Solutions

The pH of solutions is determined by using • a pH meter, • pH paper, or • indicators that have specific colors at different

pH values


Mathematically, pH is the negative logarithm (log) of the hydronium ion concentration.

pH = -log [H3O+]

For a solution with [H3O+] = 1 x 10−4

pH = −log [1 x 104 ]

pH = [4.0]

pH = 4.0

Calculating pH


Decimal Places in pH Values

When expressing pH values, the number ofdecimal places is equal to the number ofsignificant figures in the coefficient of [H3O+].

coefficient decimal places

[H3O+] = 1 x 104 pH = 4.0

[H3O+] = 8.0 x 106 pH = 5.10

[H3O+] = 2.4 x 108 pH = 7.62


Guide to Calculating pH


Find the pH of a solution with a [H3O+] of 1.0 x 10−3. STEP 1 Enter the [H3O+] value.

1.0 [EE or EXP] 3 [+/] Display: 1−03 or 103

STEP 2 Press the log key and change the sign.

[log (1 x 10−3)] [+/] = 3

STEP 3 Adjust the number of significant figures on the right of the decimal point to equal the SFs in the coefficient 1.0 x 10−3 3.00

2 SFs 2 SFs on the right of the decimal point

Example of Calculating pH


The [H3O+] of tomato juice is 2.2 x 10−4 M. What is the pH of the solution?1) 3.72) 3.663) 10.34

Learning Check


STEP 1 Enter the [H3O+] value. 2.2 [EE or EXP] 4 [+/] Display: 2.2−04 or 2.204

STEP 2 Press the log key and change the sign. log[2.2−04] [+/] = 3.657577 (Display)

STEP 3 Adjust the number of significant figures on the right of the decimal point to equal the SFs in the coefficient

2.2 x 104 3.66 (2) 2 SFs 2 SFs on the right of the decimal point

Solution


Calculating pH from [OH−]

Calculate the pH of a solution with [OH−] of 4.0 x 10−5.

STEP 1 Enter the [H3O+] value Kw = [H3O+][OH−] = 1.0 x 10−14 = 2.5 x 10 −10

[OH−] [OH−] 4.0 x 10−5

2.5 [EE or EXP] 10 [+/] Display: 2.5−10 or 2.5 10


log[2.5−10] = [9.60205] = 9.60205


Calculating pH from [OH−] (continued)

STEP 3 Adjust the number of significant figures on the right of the decimal point to equal the SFs in the coefficient.

2.5 x 10−10 M pH = 9.60 2 SFs 2SFs on the right of the decimal point


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 14.00

pH + pOH = 14.00

Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Example of Calculating pOH

Calculate the pOH of a solution that has a pH of 8.35.

pH + pOH = 14.00

pOH = 14.0 – pH = 14.00 - 8.35

= 5.65

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Calculating [H3O+] from pH

The [H3O+] can be expressed by using the pH as the negative power of 10.

[H3O+] = 10−pH

If the pH is 3.0, the [H3O+] = 1 x 10−3 M.


Example of Calculating [H3O+] from pH

Calculate the [H3O+] for a pH value of 8.0.

[H3O+] = 10−pH

STEP 1 Enter the pH value and change the sign.

8.0 [+/] Display = –8.0

STEP 2 Convert pH to [H3O+]. Press the second function key and then the 10x key or press the inverse key and then the log key.

1–08

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Example of Calculating [H3O+] from pH (continued)

STEP 3 Adjust the number of significant figures in the coefficient.

Because the pH value of 8.0 has one digit on the right of the decimal point, the [H3O+] is written with one significant figure.

For pH = 8.0, [H3O+] = 1 x 10−8 M

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What is the [H3O+] of a solution with a pH of 3.80?

Learning Check


STEP 1 Enter the pH value and change the sign. 3.80 [+/] Display = –3.80

STEP 2 Convert pH to [H3O+]. Press the second function key and then the 10x key or press the inverse key and then the log key.

3.80 (+/-) inv log (or 10x) Display = 1.5848904

STEP 3 Adjust the number of significant figures in the coefficient. Because the pH value of 3.80 has two digits on the right of the decimal point, the [H3O+] is written with two significant figures in the coefficient.

pH 3.80 [H3O+] = 1.6 x 104 M

Solution


pH Range


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


Solution

STEP 1 Enter the [H3O+] value.

1 [EE or EXP] 5 [+/] Display: 1−05 or 105


pH = –log [1 x 10−5] = –(–5.0) = 5.0


Solution (continued)

STEP 3 Adjust the number of significant figures on the right of the decimal point to equal the SFs in the coefficient.

Because the [H3O+] has one digit in the coefficient, the pH has one digit to the right of the decimal point.

pH = 5.0pH + pOH = 14.00pOH = 14.00 – pH = 14.00 – 5.0 = 9.0or [OH−] = 1 x 10−9 pOH = –log [1 x 10−9] = –(–9.0) = 9.0 (3)


What is the [H3O+] of a solution with a pOH of 11.15?

1) 1.0 x 10−2.85 M

2) 1.4 x 10−3 M

3) 8.5 x 10−2 M

Learning Check


STEP 1 Enter the pH value and change the sign. pH = 14.00 – pOH = 14.00 - 11.15 = 2.85

2.85[+/] = 2.85STEP 2 Convert pH to [H3O+].

Use second function key and then10x key or inverse key and then log key. 102.85 = 1.41253 x 10−03

STEP 3 Adjust the significant figures in the coefficient. In the pH value 2.85, there are two digits on the right of the decimal point; the [H3O+] has two significant figures.

pH = 2.85 [H3O+] = 1.4 x 10−3 M (2)

Solution

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Chapter 14 Acids and Bases