Question

Which among the following is NOT the use of $\text{SO}_2$ gas?

• A. As a preservative
• B. In manufacture of $\text{H}_2\text{SO}_4$
• C. With conc. $\text{H}_2\text{SO}_4$ it forms oleum
• D. As an antichlor

Hint:

Always carefully differentiate between the oxidation states of sulfur gases: $\text{SO}_2$ is sulfur(IV) oxide, while $\text{SO}_3$ is sulfur(VI) oxide. It is exclusively the higher oxidized state, $\text{SO}_3$, that combines with sulfuric acid to give oleum.

Answer 1

The Correct Option is C

Step 1: Read the question carefully.
We must find the statement that is NOT a real use or property of sulfur dioxide gas, $\text{SO}_2$. So three options are correct uses and one is false.

Step 2: Check option 1, preservative.
Sulfur dioxide kills germs and stops oxidation, so it is widely used to preserve dried fruits and in wine. This is a true use.

Step 3: Check option 2, making sulfuric acid.
In the Contact Process, $\text{SO}_2$ is the starting gas. It is oxidised to $\text{SO}_3$, which then becomes $\text{H}_2\text{SO}_4$. So this is a true use.

Step 4: Check option 4, antichlor.
After bleaching cloth or paper with chlorine, leftover chlorine must be removed. $\text{SO}_2$ reduces that chlorine into harmless chloride, so it works as an antichlor. This is true.

Step 5: Check option 3, oleum.
Oleum is made by adding sulfur trioxide $\text{SO}_3$ (not $\text{SO}_2$) to concentrated sulfuric acid.
\[ \text{H}_2\text{SO}_4 + \text{SO}_3 \rightarrow \text{H}_2\text{S}_2\text{O}_7 \]
Plain $\text{SO}_2$ does not combine with concentrated acid to give oleum. So this statement is false.

Step 6: Pick the false statement.
The statement that is NOT a use of $\text{SO}_2$ is the oleum one, which is option 3.
\[ \boxed{\text{With conc. } \text{H}_2\text{SO}_4 \text{ it forms oleum}} \]