Question:
When a metal ‘X’ is treated with cold water, it gives a basic salt ‘Y’ with molecular formula XOH (Molecular mass = 56) and liberates a gas ‘Z’ which
easily catches fire. Identify X, Y and Z and also write the reaction involved.
Solution:
The atomic mass of metal X = 56 – Mass of OH group = 56 — 17 = 39u. This shows that the metal ‘X’ is potassium (K) and the basic salt ‘Y’ is
potassium hydroxide (KOH). It is formed by reacting potassium with cold water. Hydrogen gas ‘Z’ evolved in the reaction catches fire.
$2 \mathrm{~K}(s)+2 \mathrm{H}_{2} \mathrm{O}(a q) \longrightarrow 2 \mathrm{KOH}(a q)+\mathrm{H}_{2}(g)$