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Q.

When concentrated sulphuric acid was added to an unknown salt present in a test tube a brown gas (A) was evolved. This gas intensified when copper turnings were added to this test tube. On cooling, the gas (A) changes into a colourless solid (B). Identify (A) and (B).


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a

<math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">A</mi><mo>:</mo><mo>&#xA0;</mo><msub><mi>NO</mi><mrow><mn>3</mn><mo>&#xA0;</mo></mrow></msub><mo>,</mo><mo>&#xA0;</mo><mi mathvariant="normal">B</mi><mo>:</mo><mo>&#xA0;</mo><msub><mi>CuNO</mi><mn>3</mn></msub></math>

b

<math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">A</mi><mo>:</mo><mo>&#xA0;</mo><msub><mi>NO</mi><mrow><msub><mn>2</mn><mrow/></msub><mo>&#xA0;</mo></mrow></msub><mo>,</mo><mo>&#xA0;</mo><mi mathvariant="normal">B</mi><mo>:</mo><mo>&#xA0;</mo><mi>Cu</mi><msub><mfenced><msub><mi>NO</mi><mn>3</mn></msub></mfenced><mn>2</mn></msub></math>

c

<math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">A</mi><mo>:</mo><mo>&#xA0;</mo><msub><mi>NO</mi><mrow><mn>3</mn><mo>&#xA0;</mo></mrow></msub><mo>,</mo><mo>&#xA0;</mo><mi mathvariant="normal">B</mi><mo>:</mo><mo>&#xA0;</mo><mi>Cu</mi><msub><mfenced><msub><mi>NO</mi><mn>3</mn></msub></mfenced><mn>2</mn></msub></math>

d

<math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">A</mi><mo>:</mo><mo>&#xA0;</mo><msub><mi>NO</mi><mrow><msub><mn>2</mn><mrow/></msub><mo>&#xA0;</mo></mrow></msub><mo>,</mo><mo>&#xA0;</mo><mi mathvariant="normal">B</mi><mo>:</mo><mo>&#xA0;</mo><msub><mi>CuNO</mi><mn>3</mn></msub></math> 

answer is B.

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Detailed Solution

concept - In a reaction between acid and salt, a more reactive metal displaces the less reactive metal from the salt and thus it is a type of displacement reaction.
Now, when sulphuric acid was added to salt, it liberated brown gas. The brown-coloured gas represents the evolution of nitrogen dioxide.
<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>MNO</mi><mrow><mn>3</mn><mo>&#xA0;</mo></mrow></msub><mo>+</mo><mo>&#xA0;</mo><msub><mi mathvariant="normal">H</mi><mn>2</mn></msub><msub><mi>SO</mi><mn>4</mn></msub><mo>&#xA0;</mo><mo>&#x2192;</mo><mo>&#xA0;</mo><msub><mi>MHSO</mi><mn>4</mn></msub><mo>&#xA0;</mo><mo>+</mo><mo>&#xA0;</mo><msub><mi>HNO</mi><mn>3</mn></msub><mspace linebreak="newline"/><mn>4</mn><msub><mi>HNO</mi><mrow><mn>3</mn><mo>&#xA0;</mo></mrow></msub><mo>&#x2192;</mo><mo>&#xA0;</mo><mn>4</mn><msub><mi>NO</mi><mrow><mn>2</mn><mo>&#xA0;</mo></mrow></msub><mo>&#xA0;</mo><mo>+</mo><mo>&#xA0;</mo><msub><mi mathvariant="normal">O</mi><mn>2</mn></msub><mo>&#xA0;</mo><mo>+</mo><mo>&#xA0;</mo><msub><mi mathvariant="normal">H</mi><mn>2</mn></msub><mi mathvariant="normal">O</mi></math>Thus, A is NO 2 .
The nitrogen gas released intensifies when copper is added:
<math xmlns="http://www.w3.org/1998/Math/MathML"><mn>3</mn><mi>Cu</mi><mo>&#xA0;</mo><mo>+</mo><mo>&#xA0;</mo><mn>8</mn><msub><mi>HNO</mi><mn>3</mn></msub><mo>&#xA0;</mo><mo>&#x2192;</mo><mo>&#xA0;</mo><mn>3</mn><mi>Cu</mi><msub><mfenced><msub><mi>NO</mi><mn>3</mn></msub></mfenced><mn>2</mn></msub><mo>&#xA0;</mo><mo>+</mo><mo>&#xA0;</mo><mn>2</mn><mi>NO</mi><mo>&#xA0;</mo><mo>+</mo><mo>&#xA0;</mo><mn>4</mn><msub><mi mathvariant="normal">H</mi><mn>2</mn></msub><mi mathvariant="normal">O</mi><mspace linebreak="newline"/><mn>2</mn><mi>NO</mi><mo>&#xA0;</mo><mo>+</mo><mo>&#xA0;</mo><msub><mi mathvariant="normal">O</mi><mrow><mn>2</mn><mo>&#xA0;</mo></mrow></msub><mo>&#xA0;</mo><mo>&#x2192;</mo><mo>&#xA0;</mo><mn>2</mn><msub><mi>NO</mi><mn>2</mn></msub></math>Thus, the colourless solid produced is Copper nitrate, thus B is Cu(NO 3 ) 2 .
Hence, the correct answer is 2.
 
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