1. CHAPTER 13
Acids
and Bases
13.2 The pH Scale

2. Soil at a high pH makes hydrangea flowers pink
Soil at a low pH makes hydrangea flowers blue

3. pH range pH can be less than 0 for stronger acids
Household chemical
Acid or base pH
ammonia
base 11
bar soap 10
baking soda
8.5
soda water
acid 4
vinegar 3
lemon juice 2
pH can be less than 0 for stronger acids
greater than 14 for stronger bases

4. pH doesn’t just tell us if a solution is neutral, an acid or a base
pH and [H+]
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

5. pH doesn’t just tell us if a solution is neutral, an acid or a base
pH and [H+]
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

6. A negative exponent means the number is less than 1
Power of 10
A negative exponent means the number is less than 1

7. pH and [H+] pH = –log[H+] Definition of pH:
Water is neutral: [H+] = 1 x 10-7 M and pH = 7
The number 7 is the logarithm of
pH = –log[H+]
Definition of pH:
Do not forget the “–” sign!
logarithm: in base 10, a number A derived from another number B such that 10B=A.

8. pH and [H+] pH = –log[H+] Definition of pH: Examples:
Water is neutral: [H+] = 1 x 10-7 M and pH = 7
The number 7 is the logarithm of
pH = –log[H+]
Definition of pH:
Examples:
[H+] = 1 M pH = –log(1) = 0
[H+] = 0.05 M pH = –log(0.05) = 1.3

9. pH and [H+] pH = –log[H+] Definition of pH: Examples:
Water is neutral: [H+] = 1 x 10-7 M and pH = 7
The number 7 is the logarithm of
pH = –log[H+]
Definition of pH:
Examples:
Check:
[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

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

11. pH and [H+]
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+]

12. pH and [H+]
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.

13. pH and [H+]
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?

14. pH and [H+]
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 = 3
100 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.

15. pH and [H+]
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 = 3
100 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.

16. pH for bases H2O(l) H+(aq) + OH–(aq) Dissociation of water:
[H+] and [OH–] are related

17. pH for bases
Find the pH of a M sodium hydroxide (NaOH) solution.

18. pH for bases
Find the pH of a M sodium hydroxide (NaOH) solution.
Asked: pH of the solution
Given: NaOH is a strong base that dissociates 100% in aqueous solution
[OH–] = M
Relationships: pH = 14 + log[OH–]

19. pH for bases
Find the pH of a M sodium hydroxide (NaOH) solution.
Asked: pH of the solution
Given: NaOH is a strong base that dissociates 100% in aqueous solution
[OH–] = M
Relationships: pH = 14 + log[OH–]
Solve: pH = 14 + log(0.012) = – 1.92 = 12.08
Answer: The solution has a pH of and is a strong base.

20. 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. 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)
The color of red cabbage juice at different pH

22. 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. 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)
- measuring the electrical properties of the solution
a pH meter
Acids and bases conduct electricity
pH and conductivity (flow of electricity) are related

24. Most acids and bases have a pH between 0 and 14
Finding the pH in acids:
pH = –log[H+]
Finding the pH in bases:
pH = 14 + log[OH–]
Measuring the pH involves indirect methods