What is the correct chemical formula for sulphurous acid, the oxyacid of sulphur in which sulphur has an oxidation state of plus four?

Introduction / Context:
This question tests your knowledge of formulas of common inorganic acids, especially oxyacids of sulphur. Sulphur forms several important acids, including sulphuric acid and sulphurous acid. Being able to distinguish their formulas is essential for writing balanced equations, understanding redox states and solving stoichiometry problems in inorganic chemistry.

Given Data / Assumptions:

• We are asked for the chemical formula of sulphurous acid.
• Sulphurous acid is an oxyacid of sulphur where sulphur is in the plus four oxidation state.
• Options include H2SO4, H2SO3, H3SO3, H3SO4 and HSO3.
• We assume standard representations used in school chemistry.

Concept / Approach:
Sulphurous acid is derived from sulphur dioxide, SO2, by hydration and is less oxidised than sulphuric acid. In sulphuric acid, sulphur has oxidation state plus six and the formula is H2SO4. In sulphurous acid, sulphur has oxidation state plus four, and the corresponding acid formula is H2SO3. Both are dibasic acids, meaning each molecule can donate two hydrogen ions in acid base reactions. The count of hydrogen atoms and oxygen atoms must be consistent with this structure and oxidation state.

Step-by-Step Solution:
Step 1: Recall that sulphuric acid has formula H2SO4 and corresponds to the higher oxidation state of sulphur, plus six. Step 2: Sulphurous acid is the acid associated with sulphur dioxide, where sulphur is in oxidation state plus four. Step 3: For sulphur in plus four state with two acidic hydrogens, the appropriate formula is H2SO3. Step 4: Check oxidation state in H2SO3. Let oxidation state of sulphur be x. Hydrogen is plus one each, oxygen is minus two. Step 5: Write the equation for the neutral molecule: 2*(+1) + x + 3*(-2) = 0. Step 6: This simplifies to 2 + x - 6 = 0, so x - 4 = 0 and x = +4, which matches sulphurous acid. Step 7: Therefore, H2SO3 is the correct formula for sulphurous acid.

Verification / Alternative check:
Another way to verify is to remember that sulphurous acid is dibasic, so it must contain two hydrogen atoms. This rules out HSO3 immediately. Then, comparing H2SO3 and H2SO4, you know that H2SO4 is the well known sulphuric acid. That leaves H2SO3 as sulphurous acid. Formulas like H3SO3 or H3SO4 would indicate a tribasic acid with three hydrogen ions, which is not how these sulphur oxyacids are classified in standard chemistry texts.

Why Other Options Are Wrong:
H2SO4 is sulphuric acid, not sulphurous acid, and corresponds to sulphur in oxidation state plus six. H3SO3 and H3SO4 would imply three acidic hydrogens and are not standard formulas for known simple sulphur oxyacids at this level. HSO3 is an anion form, the hydrogen sulphite ion, not the neutral acid molecule. Therefore, these options do not correctly represent sulphurous acid.

Common Pitfalls:
Students often confuse sulphuric acid and sulphurous acid because their names are very similar and differ only by the suffix. A useful memory aid is that the more oxidised acid, sulphuric, has more oxygen atoms (H2SO4), while the less oxidised sulphurous acid has fewer oxygen atoms (H2SO3). Another pitfall is to forget that these acids are dibasic, leading to incorrect hydrogen counts in proposed formulas. Keeping track of both oxidation state and basicity helps avoid such errors.

Final Answer:
The correct chemical formula for sulphurous acid is H2SO3.