10.4 the p_h_scale

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 1

Chapter 10 Acids and Bases

10.4The 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, 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


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


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


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


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


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:

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


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


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 )


log (1 x 10−5) = [5]


[H3O+]= 1 x 10−5, pH is 5.0 (3)


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


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 )


log (2 x 10−4) = [3.7]


[H3O+] = 2 x 10−4, pH is = 3.7 (2)

Solution


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


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)


log (1.0 x 10−11) = [11]


[H3O+] =1.0 x 10−11, pH is = 11.00 (2)

Solution


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


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


Example 2: Calculating [H3O+] from pHCalculate the [H3O+] for a pH of 3.80.



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


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





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



1) 1.0 x 10−2.85 M

2) 1.4 x 10−3 M

3) 8.5 x 10−2 M

Learning Check





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

10.4 the p_h_scale

Technology

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