CHAPTER 13

Acids and Bases

13.2 The pH Scale

2 13.2 The pH Scale

Soil at a high pH makes hydrangea flowers pink

Soil at a low pH makes hydrangea flowers blue

3 13.2 The pH Scale

Household chemical

Acid or base pH

ammonia base 11

bar soap base 10

baking soda base 8.5

soda water acid 4

vinegar acid 3

lemon juice acid 2

pH range

pH can be less than 0 for stronger acidsgreater than 14 for stronger bases

4 13.2 The pH Scale

pH doesn’t just tell us if a solution is neutral, an acid or a base

It also tells us:

the concentration of H+ ions in the solution in moles/L

Water is neutral: [H+] = 1 x 10-7 M and pH = 7

pH and [H+]

5 13.2 The pH Scale

pH doesn’t just tell us if a solution is neutral, an acid or a base

It also tells us:

the concentration of H+ ions in the solution in moles/L

which is expressed as a power of 10

Water is neutral: [H+] = 1 x 10-7 M and pH = 7

pH and [H+]

6 13.2 The pH Scale

Power of 10

A negative exponent means the number is less than 1

7 13.2 The pH Scale

Definition of pH: pH = –log[H+]

Water is neutral: [H+] = 1 x 10-7 M and pH = 7

Do not forget the “–” sign!

logarithm: in base 10, a number A derived from another number B such that 10B=A.

The number 7 is the

logarithm of 0.000 000 1

pH and [H+]

8 13.2 The pH Scale

[H+] = 1 M pH = –log(1) = 0

[H+] = 0.05 M pH = –log(0.05) = 1.3

Examples:

Definition of pH: pH = –log[H+]

Water is neutral: [H+] = 1 x 10-7 M and pH = 7 The number 7 is the

logarithm of 0.000 000 1

pH and [H+]

9 13.2 The pH Scale

[H+] = 1 M pH = –log(1) = 0 [H+] = 10–pH = 10–0 = 1 M

[H+] = 0.05 M pH = –log(0.05) = 1.3 [H+] = 10–pH = 10–1.3 = 0.05 M

Examples:Check:

Definition of pH: pH = –log[H+]

Water is neutral: [H+] = 1 x 10-7 M and pH = 7 The number 7 is the

logarithm of 0.000 000 1

pH and [H+]

10 13.2 The pH Scale

A solution of acetic acid (HCH3O2) has an H+ concentration of 5 x 10–5 M. What is the pH of the solution?

pH and [H+]

11 13.2 The pH Scale

A solution of acetic acid (HCH3O2) has an H+ concentration of 5 x 10–5 M. What is the pH of the solution?

Asked: pH of a solution

Given: [H+] = 5 x 10–5 M

Relationships: pH = –log[H+]

pH and [H+]

12 13.2 The pH Scale

A solution of acetic acid (HCH3O2) has an H+ concentration of 5 x 10–5 M. What is the pH of the solution?

Asked: pH of a solution

Given: [H+] = 5 x 10–5 M

Relationships: pH = –log[H+]

Solve: pH = –log[H+]

pH = –log(5 x 10–5)

pH = 4.3

Answer: This solution has a pH of 4.3, a relatively weak acid.

pH and [H+]

13 13.2 The pH Scale

A solution of nitric acid (HNO3) has a pH of 3. What will the pH be if you add 10 mL of the solution to 90 mL of pure water?

pH and [H+]

14 13.2 The pH Scale

A solution of nitric acid (HNO3) has a pH of 3. What will the pH be if you add 10 mL of the solution to 90 mL of pure water?

Asked: pH of the new solution

Given: old pH = 3100 mL of the new solution contains 10 mL of the old solution

Relationships: A pH value is a power of 10.A change in 1 pH unit means the concentration changes by a

factor of 10.

pH and [H+]

15 13.2 The pH Scale

A solution of nitric acid (HNO3) has a pH of 3. What will the pH be if you add 10 mL of the solution to 90 mL of pure water?

Asked: pH of the new solution

Given: old pH = 3100 mL of the new solution contains 10 mL of the old solution

Relationships: A pH value is a power of 10.A change in 1 pH unit means the concentration changes by a

factor of 10.

Solve: Diluting an acidic solution means the pH increases (fewer H+)The new pH is 4 (not 2).

Answer: The new solution has a pH of 4.

pH and [H+]

16 13.2 The pH Scale

Dissociation of water:

H2O(l) H+(aq) + OH–(aq)

[H+] and [OH–] are related

1410

14

14

concentration of H concentration of OH

log H log OH

pH log OH

pH for bases

17 13.2 The pH Scale

Find the pH of a 0.012 M sodium hydroxide (NaOH) solution.

pH for bases

18 13.2 The pH Scale

Find the pH of a 0.012 M sodium hydroxide (NaOH) solution.

Asked: pH of the solution

Given: NaOH is a strong base that dissociates 100% in aqueous solution

[OH–] = 0.012 M

Relationships: pH = 14 + log[OH–]

pH for bases

19 13.2 The pH Scale

Find the pH of a 0.012 M sodium hydroxide (NaOH) solution.

Asked: pH of the solution

Given: NaOH is a strong base that dissociates 100% in aqueous solution

[OH–] = 0.012 M

Relationships: pH = 14 + log[OH–]

Solve: pH = 14 + log(0.012) = 14.00 – 1.92 = 12.08

Answer: The solution has a pH of 12.08 and is a strong base.

pH for bases

20 13.2 The pH Scale

Measuring pH

You can’t measure pH by just looking at a solution, or measuring its density

or temperature, but you can measure pH indirectly by:

- performing a chemical reaction with a solution of known pH

21 13.2 The pH Scale

Measuring pH

The color of red cabbage juice at different pH

You can’t measure pH by just looking at a solution, or measuring its density

or temperature, but you can measure pH indirectly by:

- performing a chemical reaction with a solution of known pH

- using a chemical that changes color at different pH values

(pH indicators)

22 13.2 The pH Scale

Measuring pH

You can’t measure pH by just looking at a solution, or measuring its density

or temperature, but you can measure pH indirectly by:

- performing a chemical reaction with a solution of known pH

- using a chemical that changes color at different pH values

(pH indicators)

23 13.2 The pH Scale

You can’t measure pH by just looking at a solution, or measuring its density

or temperature, but you can measure pH indirectly by:

- performing a chemical reaction with a solution of known pH

- using a chemical that changes color at different pH values (pH indicators)

- measuring the electrical properties of the solution

Measuring pH

Acids and bases conduct electricity

pH and conductivity (flow of electricity) are related

a pH meter

24 13.2 The pH Scale

pH = –log[H+]

pH = 14 + log[OH–]

Finding the pH in acids:

Finding the pH in bases:

Measuring the pH involves indirect methods

Most acids and bases have a pH between 0 and 14