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Q.

A soft metal (A) gives a violet flame test. When burnt in excess <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi mathvariant="normal">O</mi><mn>2</mn></msub></math>, (A) gives chrome yellow powder (B) which on reaction with water gives alkaline solution (C) and <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi mathvariant="normal">O</mi><mn>2</mn></msub></math>, (A) gives chrome yellow powder (B) which on reaction with water gives alkaline solution (C) and . Identify A and B.


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a

<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>Na</mi><mo>,</mo><mo>&#xA0;</mo><msub><mi>Na</mi><mn>2</mn></msub><msub><mi mathvariant="normal">O</mi><mn>2</mn></msub></math>

b

<math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">K</mi><mo>,</mo><mo>&#xA0;</mo><msub><mi mathvariant="normal">K</mi><mn>2</mn></msub><msub><mi mathvariant="normal">O</mi><mn>2</mn></msub></math>

c

<math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">K</mi><mo>,</mo><mo>&#xA0;</mo><msub><mi>KO</mi><mn>2</mn></msub></math>

d

<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>Li</mi><mo>,</mo><mo>&#xA0;</mo><msub><mi>Li</mi><mn>2</mn></msub><msub><mi mathvariant="normal">O</mi><mn>2</mn></msub></math> 

answer is C.

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Detailed Solution

concept - All alkali metals (Lithium, sodium, potassium, Rubidium and Cesium) are soft and can be cut by a knife.
Alkali metals react with oxygen to produce their oxides.
Lithium oxide is the lightest alkali metal oxide and a white solid.
Sodium oxide is a white solid
Potassium oxide is a pale-yellow solid.
Rubidium oxide is a yellow solid.
Cesium oxide is a yellow-orange solid.
Now, a soft metal which gives violet flame test is potassium, K.
<math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal">K</mi><mo>&#xA0;</mo><mo>+</mo><mo>&#xA0;</mo><mi>excess</mi><mo>&#xA0;</mo><msub><mi mathvariant="normal">O</mi><mrow><mn>2</mn><mo>&#xA0;</mo></mrow></msub><mo>&#x2192;</mo><mo>&#xA0;</mo><msub><mi>KO</mi><mn>2</mn></msub></math>So, B is potassium oxide.
<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>KO</mi><mrow><mn>2</mn><mo>&#xA0;</mo></mrow></msub><mo>+</mo><mo>&#xA0;</mo><msub><mi mathvariant="normal">H</mi><mn>2</mn></msub><mi mathvariant="normal">O</mi><mo>&#xA0;</mo><mo>&#x2192;</mo><mo>&#xA0;</mo><mn>4</mn><mi>KOH</mi><mo>&#xA0;</mo><mo>+</mo><mo>&#xA0;</mo><mn>3</mn><msub><mi mathvariant="normal">O</mi><mn>2</mn></msub></math>So, C is potassium hydroxide.
Hence, the correct answer is 3.
 
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