A rock sample was added to cold dilute HNO3. The gas evolved was passed into a solution of acidified K2Cr2O7 and the solution turned green. The sample contains
Cl-
NO3-
SO32-
SO42-
Explanation
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A common characteristic of trioxosulphate(iv) salts is that they react with dilute acids to produce sulphur(iv) oxide. In fact this reaction is used to test for trioxosulphate(iv) salts.
SO₃²⁻ + 2H⁺ --> SO₂(g) + H₂O(l)
Sulphur(iv) oxide is a strong reducing agent, hence it changes the colour of acidified K₂Cr₂O₇ from orange to green
Cr₂O₇²⁻ + 3SO₂ +2H⁺ --> 2Cr³⁺ + 3SO₄²⁻ + H₂O
Here's how we can figure out the answer:
* HNO3 (nitric acid) and Gas Evolution: When a rock sample reacts with cold, dilute nitric acid (HNO3), the gas that comes out is usually related to a reaction with something that can be oxidized.
* Gas and Acidified K2Cr2O7: Acidified potassium dichromate (K2Cr2O7) turning green is a classic test for sulfur dioxide (SO2). This means the gas evolved must have been SO2.
* Relating SO2 to the Sample: SO2 comes from sulfite ions (SO_3^2-)
Therefore, the sample contains C. SO32-.
