This document discusses the differences between strong acids and weak acids in aqueous solutions and calculating pH. It states that strong acids, like HCl, completely dissociate in water, resulting in hydronium ion concentrations equal to the original acid concentration. Therefore, the pH of a strong acid solution can be calculated directly from its concentration. Weak acids, like HF, exist in an equilibrium with their dissociated and undissociated forms. Their pH cannot be determined directly and requires calculating equilibrium concentrations. The document gives the example of calculating the pH of a 0.05 M HI solution as 1.30 since HI is a strong acid.

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)