Ncert Exercises & Solutions

An amorphous solid“A” burns in air to form a gas “B” which turns lime water milky. The gas is also produced as a by-product during roasting of sulphide ore. This gas decolourises acidified aqueous KMnO₄, solution and reduces Fe³⁺to Fe²⁺. Identify the solid “A” and the gas “B”and write the reactions involved.

Since, the by-product of roasting of sulphide ore is SO₂so A is S₈,so 'A' = S₈: ‘B'= SO₂

Reactions

$(i) S_{8} + 8 O_{2} \overset{∆}{\to} 8 {SO}_{2}$
$(ii) Ca {(OH)}_{2} + {SO}_{2} \to {CaSO}_{3} + H_{2} O$
$(iii) \underset{(Violet)}{2 {MnO}_{4}^{-}} + 5 {SO}_{2} + 2 H_{2} O \to 5 {SO}_{4}^{2 -} + 4 H^{+} + \underset{(Colourless)}{2 {Mn}^{2 +}}$
$(iv) 2 {Fe}^{+ 3} + {SO}_{2} + 2 H_{2} O \to 2 {Fe}^{^{2 -}} + {SO}_{4}^{2 -} + 4 H^{+}$

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