1. Section 6.3—Acidity, pH Objectives
How does concentration of acid affect the pH of a sports drink?
Objectives
Calculate the pH from the concentration of hydronium ions and vice versa
Explain the connection between strength and pH
Calculate the concentration of hydroxide ions from hydronium ions and vice versa

2. Review of Acids – Arrhenius Definition
Acids produce a hydronium ion (H3O+1) in water
A hydronium ion is water + a hydrogen cation H O
water H O +1 H +1
By this definition, if an acid is to give a H+1 to water, then all acids will have hydrogen as the cation (first element written).

3. How do Acids produce Hydronium?
Cl-
water
hydrochloric acid
Hydrogen cation (H+) with Cl- anion

4. How do Acids produce Hydronium?
Cl- +1 H O

5. How do Acids produce Hydronium?
Cl- +1 H O
Hydronium ion
Chloride anion

6. Bases – Arrhenius Definition
Bases produce the hydroxide ion in water H O -1
Hydroxide Ion

7. Characteristics of Acids & Bases
Produce H3O+1 (hydronium ion) in water
Produce OH-1 (hydroxide ion) in water
Tastes sour
Tastes Bitter
React with most metals to form hydrogen gas
Feels slippery

8. Strong versus Weak Acids
How many hydronium ions can you find? How many anions can you find? - + - 3 +
How many intact acid molecules can you find? + - 1
Strong acid
MOST of the acid molecules have donated their H+ to water

9. Strong versus Weak Acids+
How many hydronium ions can you find? How many anions can you find? 1
How many intact acid molecules can you find? - 3
Weak acid
Only a FEW of the acid molecules have donated their H+ to water

10. Concentrated versus Dilute
solvent
solute
Lower concentration
Only a FEW solute particle dissolved
Higher concentration
A LOT of solute particles dissolved

11. Combinations of Concentration & Strength
Concentrated
Dilute
Strong
A lot of acid added & most dissociates (forms ions)
Not much acid added, but most of what’s there dissociates (forms ions)
Weak
A lot of acid added, but most stays together
Not much acid added and most of what is there stays together

12. Acids and Bases as Electrolytes
Acids and bases dissociate in ions in water
Free-floating ions in water conduct electricity
Acids & Bases are electrolytes
Strong acids and bases are strong electrolytes.
Weak acids and bases are weak electrolytes.

13. pH
S.P.L. Sorenson developed the pH scale in pH stands for “power of hydrogen”.
[H+] (the concentration of H+) is not expressed as a small number in scientific notation but as a logarithm.

14. pH pH is defined as the negative logarithm of the [H+] or - log [H+].
Think of it this way: the pH of a solution is the negative of the exponent on 10 that gives us the [H+].
For example, if a solution has [H+] = M, its pH is -(the exponent on 10), which is -(-3) or 3.

15. pH
This means as the concentration of H+ increases, pH decreases!! So . . .
Acids have a pH from 0 to below 7. The lower the number, the stronger the acid.
Neutral (not acidic or basic) is at 7.
Bases have a pH from above 7 to 14. The higher the number, the stronger the base.

16. Are these solution acidic, basic, or neutral?
Practice Problems
Calculate the pH of solutions having the following ion concentrations.
[H+] = 1 x 10-6 M
[H+] = 1 x 10-7 M
[H+] = 1 x M
Are these solution acidic, basic, or neutral?

17. More Practice
What is the [H+] if the pH is 2 13 8

18. What does a “log” scale really mean?
Every change of 1 in pH shows a change of 10x
in concentration of hydronium. pH 4 3 2 1
Level of acidity increases
1000x more acidic
100x more acidic
10x more acidic

19. Auto-ionization of Water
Water molecules will split into their ions:
2 H2O  H3O+1 + OH-1 OR
H2O  H+ + OH-
Studies have shown that at 25°C the following is true:
[H3O+1] [OH-1] = 10-14
This means that if you know the hydronium concentration at 25°C (which can be found from pH), then you can also find the hydroxide concentration

20. Practice Problems
The [H+] of an aqueous solution is 1 x 10-5 M. What is the [OH-]? Is the solution acidic, basic or neutral?
If the [OH-] of an aqueous solution is 1 x 10-3 M, what is the [H+]? Is the solution acidic, basic, or neutral?
Find the [OH-] if the pH is 13.

21. pOH pOH is the negative logarithm of the [OH-] or -log [OH-].
Therefore, a solution with [OH-] = 1 x 10-2 M has a pOH of 2.
A simple relationship between pH & pOH makes it simple to calculate one when the other is known:
pH + pOH = 14

22. Practice Problems
If the [OH-] = 1 x 10-3 M, what is the pOH and pH of the solution? Is the solution acidic, basic, or neutral?
A solution has a pH of 8. What are the [H+] & [OH-]? Is the solution acidic, basic, or neutral?
A solution has a pOH of 4. What are the [H+] & [OH-]? Is the solution acidic, basic, or neutral?