Question

In a test tube containing a salt, a few drops of dilute \({H2SO4}\) was added, which gave colourless vapours having the smell of vinegar. The vapours turned blue litmus paper red. Identify the correct anion from the following.

• A. Carbonate, \({CO3^{2-}}\)
• B. Sulphite, \({SO3^{2-}}\)
• C. Acetate, \({CH3COO^-}\)
• D. Sulphide, \({S^{2-}}\)

Hint:

Acetate salts give vinegar-smelling acetic acid vapours with dilute sulphuric acid.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
This is a question regarding qualitative inorganic analysis. We are observing the reaction between an unknown salt and dilute sulfuric acid. The identifying characteristics provided are the color of the evolved vapors (colorless), their distinct smell (vinegar), and their effect on litmus paper (acidic). We need to correlate these observations with the standard reactions of specific anions.
Step 2: Key Formula or Approach:
The approach involves knowing the gases produced when dilute acids react with specific anionic radicals:
Carbonate ($CO_3^{2-}$): Produces $CO_2$ (colorless, odorless, turns lime water milky).
Sulphide ($S^{2-}$): Produces $H_2S$ (colorless, smell of rotten eggs).
Acetate ($CH_3COO^-$): Produces Acetic acid vapors (colorless, smell of vinegar).
Sulphite ($SO_3^{2-}$): Produces $SO_2$ (colorless, smell of burning sulfur).
Step 3: Detailed Explanation:

The Smell of Vinegar: The most defining characteristic in this problem is the "smell of vinegar." In chemistry, this odor is exclusively associated with acetic acid ($CH_3COOH$).
Reaction with Acid: When dilute $H_2SO_4$ is added to an acetate salt (like sodium acetate), the acetate ion is protonated to form acetic acid. \[ 2CH_3COONa + H_2SO_4 \rightarrow Na_2SO_4 + 2CH_3COOH \]
Acidic Nature: Acetic acid is a volatile weak acid. When it escapes as vapor, it remains acidic. Therefore, when these vapors come into contact with moist blue litmus paper, they turn it red, which is a standard test for acids.
Excluding Other Options: Sulphide gives a rotten egg smell ($H_2S$). Carbonate gives odorless $CO_2$. Sulphate ($SO_4^{2-}$) does not react with dilute $H_2SO_4$ to give any gas, as it is the ion from the acid itself.
Step 4: Final Answer:
Based on the specific vinegary odor and the acidic nature of the vapors, the anion is Acetate ($CH_3COO^-$).