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.

Alkali metals react with water to give a base. In our case, the molecular weight of this compound is given to be 40. If the base is of the formula XOH, where X is the alkali metal, we have,

MW of XOH = x+16+1 = 40
x=23

The alkali metal with atomic weight 23 is sodium (Na).
[1 Mark]

If we assume that the base is of higher basicity, the value 40 will be exceeded.Thus, A would be sodium and B, Sodium hydroxide.
[1 Mark]

For the 2nd reaction, we have Aluminium oxide reacting with sodium hydroxide.Aluminium oxide is an amphoteric oxide and can react in both basic and acidic manner. Here we have NaOH, which is a base. $A{l}_2{O}_3$ will hence act as an acidic salt. The reaction will proceed similar to an acid-base reaction, giving a salt and water as the product.
[1 Mark]

$A-Na;B-NaOH;C-NaAl{O}_2$

$2Na+2H_{2}O\to 2NaOH+H_2$
[1 Mark]

$A{l}_2{O}_3+2NaOH⟶2NaAl{O}_2+H_{2}O$
[1 Mark]