Question

An amorphous solid $A$ burns in air to form a gas $B$ which turns lime water milky. This gas is also produced as a by-product during reacting of sulphide ore. This gas decolourises acidified aqueous $KMn{O}_4$ solution and reduces $F{e}^{3+}$ to $F{e}^{2+}$. Identify the solid $A$ and the gas $B$ and write the reactions involved.

Answer 1

A is $S_8$ and B is $S{O}_2$ gas.

$S_8+8O_2\stackrel{\Delta }{\to }8S{O}_2$

$2Mn{O}_4^{-}+5S{O}_2+2H_{2}O\to 5S{O}_4^{2-}+4H^{+}+2M{n}^{2+}$

(violet) (colorless)

$2F{e}^{3+}+S{O}_2+2H_{2}O\to 2F{e}^{2+}+S{O}_4^{2-}+4H^{+}$