Chem 2 - Acid-Base Equilibria IV: Calculating the pH of Strong Acids versus Weak Acid Equilibria

1. Acid-Base Equilibria (Pt. 4)
Calculating the pH of Strong
Acids versus Weak Acid
Equilibria
By Shawn P. Shields, Ph.D.
This work is licensed by Dr. Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0
International License.

2. Recall: Strong versus Weak Acids
Strong acids dissociate completely in
solution.
𝐇𝐂𝐥 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 → 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐂𝐥−
(𝐚𝐪)
Weak acids only partially dissociate in
solution.
𝐇𝐅 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐅−
(𝐚𝐪)

3. Strong Acids in Aqueous Solution
𝐇𝐂𝐥 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 → 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐂𝐥−
(𝐚𝐪)
All of the strong acid (HCl) dissociates
completely
The concentration of H3O+ in solution is
equal to the original concentration of
the strong acid.

4. Strong Acid Solutions
Suppose a 1.0 M solution of HCl
is prepared.
What is the concentration of
hydronium (H3O+) in solution?
What is the concentration of HCl
(undissociated acid) in solution?
H3O+
Cl

5. Strong Acid Solutions
Suppose a 1.0 M solution of HCl
is prepared.
What is the concentration of
hydronium (H3O+) in solution?
1.0 M
What is the concentration of HCl
(undissociated acid) in solution?
(essentially) zero
H3O+
Cl

6. Recall: The pH Scale
The acidity of a substance can be
determined by measuring the
concentration of hydronium H3O+ in
solution.
pH is related to the concentration of
hydronium H3O+.
pH =  log [H3O+]

7. Calculate the pH of a Strong Acid Solution
Suppose a 0.05 M HI solution is prepared.
What is the pH of this solution?
(HI is a strong acid.)
𝐇𝐈 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 → 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐈−
(𝐚𝐪)
pH =  log [H3O+] =  log [0.05] = 1.30
0After
dissolution
0.05 M 0.05 M

8. Compare: Weak Acid Solutions
Suppose a 1.0 M solution of HF
(a weak acid) is prepared.
What is the concentration of
hydronium (H3O+) in solution?
What is the concentration of HF
(undissociated acid) in solution?
H3O+
F
HF

9. Compare: Weak Acid Solutions
Suppose a 1.0 M solution of HF (a weak
acid) is prepared.
What is the concentration of hydronium
(H3O+) in solution?
We can’t do this by inspection.
An equilibrium between the weak
acid and the products exists.
We need to find [H3O+] by calculating
the equilibrium concentrations of HF,
H3O+, and F-
H3O+
F
HF

10. What you Should Be Able to Do
Calculate the pH of a strong acid
solution.
Identify strong acids upon
inspection (memorize the list given).
Write the chemical reaction for
strong and weak acids in water.

11. Next up,
Weak Acid Equilibria and pH
(Pt 5)