Among the following common acids, which one is not classified as a strong acid in aqueous solution?

Introduction / Context:
Acids can be classified as strong or weak based on how completely they ionise in water. Strong acids ionise almost completely, while weak acids ionise only partially. This question asks you to identify which listed acid is not strong, meaning it does not fully ionise in aqueous solution.

Given Data / Assumptions:
- The acids listed are sulfuric acid, acetic acid, hydrochloric acid, hydrobromic acid, and nitric acid.
- In introductory chemistry, a short list of strong acids is commonly memorised.
- Only one of the acids given is treated as a weak acid in most school level chemistry courses.

Concept / Approach:
Strong acids at this level usually include HCl, HBr, HI, HNO3, HClO4, and H2SO4. These acids almost completely dissociate into ions in water. Acetic acid CH3COOH, the main acid component of vinegar, is a weak acid and only partially ionises. Its acid dissociation constant is much smaller than that of the strong acids, so a significant fraction of CH3COOH molecules remain undissociated in solution.

Step-by-Step Solution:
Step 1: Recall the standard list of strong acids: HCl, HBr, HI, HNO3, HClO4, and H2SO4. Step 2: Compare the list in the question and note that sulfuric acid, hydrochloric acid, hydrobromic acid, and nitric acid all appear in the strong acid list. Step 3: Recognise that acetic acid CH3COOH is commonly used as an example of a weak acid. Step 4: Understand that acetic acid does not completely dissociate in water; instead it establishes an equilibrium between CH3COOH and its ions. Step 5: Therefore, acetic acid CH3COOH is the acid that is not classified as strong in aqueous solution.

Verification / Alternative check:
You can compare the pH of equimolar solutions. A 0.1 molar solution of HCl or HNO3 shows a pH close to 1, reflecting almost complete ionisation. In contrast, a 0.1 molar solution of acetic acid has a higher pH, closer to 2.9 or 3, because only a fraction of the molecules ionise. Additionally, weak acids like acetic acid have equilibrium expressions and buffer behaviour, reinforcing their partial dissociation character.

Why Other Options Are Wrong:
- Sulfuric acid H2SO4: A well known strong acid, especially in its first ionisation step, widely used in industry and laboratories.
- Hydrochloric acid HCl: A classic strong acid that dissociates almost entirely into H+ and Cl− ions in water.
- Hydrobromic acid HBr: Another halogen based strong acid that fully ionises in aqueous solution.
- Nitric acid HNO3: A strong acid used in fertiliser production and nitration reactions, which dissociates extensively in water.

Common Pitfalls:
Learners sometimes assume that any acid with a familiar name is strong, or that organic acids such as acetic acid are strong because they have a pronounced sour taste. However, taste is not a reliable measure of strength, and many organic acids are weak. Memorising the common strong acids and remembering that most other acids, especially many carboxylic acids, are weak helps to answer such questions quickly.

Final Answer:
The acid that is not classified as strong in aqueous solution is Acetic acid CH3COOH.