An alkali metal A gives a compound B (molecular mass = 40) on reacting with water. The compound B gives a soluble compound C on treatment with aluminium oxide. Identify A, B and C and give the reaction involved

NCERT Class X
Science - Exemplar Problems

Chapter_Metals and Non Metals

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2015-05-29T10:03:57+05:30

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Alkali metals are    sodium, potassium, lithium etc.

On reacting with water , they give metal hydroxide..  M_k (OH)_n

Hydroxide molecular mass = 17..
As molecular mass of the metal hydroxide is 40,  n = 1 or 2,  and  correspondingly the molecular mass of metal is  23 or 6.
That is metal A is  either sodium or lithium.

Compound B is  :  sodium hydroxide or  lithium hydroxide..  Lithium hydroxide - molecular mass is not 40.. 

So metal A is Sodium : Na...    compound B is sodium hydroxide.
   aluminum oxide:   Al₂ O₃ + 2 Na OH  +3 H₂O ==> 2 Na Al (OH)₄
          
Compound C :    colourless solution in water ... of  compound : sodium tetra aluminate..

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