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Chapter 8 Acids and Bases

8.5 The pH Scale

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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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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pH of Everyday Substances

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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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

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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 • a) pH meter• b) pH paper• c) indicators that have specific colors at different pH

values.

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pH is the negative log of the hydronium ion concentration.pH = - log [H3O+]

Example: For a solution with [H3O+] = 1 x 10−4 M

pH = −log [1 x 10−4 M]pH = - [-4.0]pH = 4.0

Note: The number of decimal places in the pH equals the significant figures in the coefficient of [H3O+].

4.0 1 SF in 1 x 10-4 M

Calculating pH

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pOH is the negative log of the hydroxide ion concentration.pOH = - log [OH−]

Example: For a solution with [OH−] = 1 x 10−12 M

pH = −log [1 x 10−12 M]pH = - [-12.0]pH = 12.0

Note: The number of decimal places in the pOH equals the significant figures in the coefficient of [OH−].

12.0 1 SF in 1 x 10-12 M

Calculating pOH

Note: pKw = 14 = pH + pOH

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Guide to pH or pOH Calculations

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Significant Figures in pH and pOH

When expressing log values, the number of decimalplaces in the pH or pOH is equal to the number of significant figures in the coefficient of [H3O+] or [OH−].

[H3O+] = 1 x 10-4 pH = 4.0

[H3O+] = 8.0 x 10-6 pH = 5.10

[OH−] = 1 x 10-12 pOH = 12.0

[OH−] = 2.4 x 10-8 pOH = 7.62

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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 figures after decimal point to equal the significant figures in the coefficient.

3 3.00 Two significant figures in 1.0 x 10−3

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 10−5M?

3) pH = 5.0pH = -log [1 x 10−5] = -(-5.0) = 5.0

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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) 3.7pH = - log [ 2 x 10-4] = 3.72 (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

[ 1 x 10-14]

[ 1 x 10-3]

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[H3O+], [OH-], and pH Values

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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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

For pH = 3.0, the [H3O+] = 1 x 10 -3 MOn a calculator

1. Enter the pH value 3.02. Change sign -3.03. Use the inverse log key (or 10x) to obtain

the [H30+]. = 1 x 10 -3 M

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Calculating [OH−] from pOH

The [OH−] can be expressed by using the pOH as the negative power of 10.

[OH−] = 1 x 10 -pOH

For pOH = 10.0, the [OH−] = 1 x 10 -10 MOn a calculator

1. Enter the pOH value 10.02. Change sign -10.03. Use the inverse log key (or 10x) to obtain

the [OH−]. = 1 x 10 -10 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 [OH−] of a solution with a pH of 2.00? 1) 1.0 x 10− 2 M 2) 1.0 x 10−12 M 3) 2.0 M

Learning Check

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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 [OH−] of a solution with a pH of 2.00? 2) 1.0 x 10−12 M

[H3O+] = 1.0 x 10−2 M 1 x 10-pH

[OH−] = 1.0 x 10−14 M2 = 1.0 x 10−12 M, use Kw!

1.0 x 10−2 M

Solution