Question:which one of the following is a weak acid
Which One of the Following Is a Weak Acid?
Question
Which one of the following is a weak acid?
1. What Is a Weak Acid?
A weak acid is an acid that does not completely ionize when dissolved in water. Instead, an equilibrium is established between the undissociated acid and its ions. :contentReference[oaicite:0]{index=0}
The double arrow indicates that the ionization is reversible and incomplete.
2. Why Is Acetic Acid a Weak Acid?
Acetic acid has the chemical formula CH₃COOH. In water, it establishes an equilibrium:
Only a fraction of the molecules ionize. This partial ionization is the key reason acetic acid is classified as a weak acid. Its acid dissociation constant is relatively small, about 1.8 × 10⁻⁵ at room temperature. :contentReference[oaicite:1]{index=1}
3. Option A — HCl
HCl is hydrochloric acid. It is classified as a strong acid because it ionizes essentially completely in aqueous solution. :contentReference[oaicite:2]{index=2}
Therefore:
HCl is not the correct answer. ❌
4. Option B — CH₃COOH
CH₃COOH is acetic acid, also called ethanoic acid. It is a classic example of a weak acid. :contentReference[oaicite:3]{index=3}
It ionizes only partially:
The equilibrium arrow is important because the reaction does not proceed completely to the right.
5. Option C — H₂SO₄
H₂SO₄ is sulfuric acid. Its first ionization in water is treated as essentially complete, so it is classified as a strong acid in general introductory chemistry. :contentReference[oaicite:4]{index=4}
Therefore, H₂SO₄ is not the weak acid in this question.
6. Option D — NaOH
NaOH stands for sodium hydroxide. It is a base, not an acid.
Therefore, it cannot be the answer to a question asking for a weak acid.
7. Comparison of All Options
| Option | Compound | Type | Answer? |
|---|---|---|---|
| A | HCl | Strong acid | ❌ |
| B | CH₃COOH | Weak acid | ✅ |
| C | H₂SO₄ | Strong acid | ❌ |
| D | NaOH | Strong base | ❌ |
8. Strong Acid vs Weak Acid
9. Important Concept: Strength vs Concentration
Acid strength and acid concentration are different concepts.
Strength refers to how extensively an acid ionizes in water.
Concentration refers to how much acid is present in a given volume of solution.
Therefore, a concentrated weak acid can still be a weak acid, while a dilute strong acid remains a strong acid in terms of its intrinsic ionization behavior.
10. Common Examples of Weak Acids
Common weak acids include acetic acid, formic acid, carbonic acid and citric acid. :contentReference[oaicite:5]{index=5}
11. Quick Exam Trick
Strong acid → almost complete ionization
Weak acid → partial ionization
CH₃COOH = Acetic acid = Weak acid
12. Detailed Final Solution
Why is B correct?
CH₃COOH is acetic acid. It is a weak acid because it partially ionizes in water rather than completely dissociating into ions.
HCl and H₂SO₄ are treated as strong acids in standard introductory chemistry, while NaOH is a strong base. :contentReference[oaicite:6]{index=6}
Final Summary
The correct answer to the question “Which one of the following is a weak acid?” is:
Acetic acid is weak because it only partially ionizes in aqueous solution. The equilibrium nature of its ionization can be represented by:
HCl → Strong Acid
CH₃COOH → Weak Acid ✅
H₂SO₄ → Strong Acid
NaOH → Strong Base