Question

A mixture of treads was treated as follows 

(i) The mixture was heated with manganese dioxide and concentrated sulphuric acid, when yellowish green gas was liberated. 

(ii) The mixture on heating with sodium hydroxide solution tave a gas which turned red litmus blue. 

(iii) Its solution in water gave blue precipitate potassium ferricyanide and red colouration ammonium thiocyanate. 

(iv) The mixture was boiled with potassium hydroxide and the liberated gas was bubbled through an alkaline solution of K₂Hg I₄ to give brown precipitate. 

Identify the for salts. Give ionic equations for reactions involved in the tests (i), (ii) and (iii).

Answer 1

(i) Mixture + MnO₂ + H₂SO₄ (conc.) → yellowish greengas

The reaction suggests that the mixture contains Cl^- ions.

(ii) Mixture + NaOH → Gas turning red litmus blue The reaction suggests that the gas is ammonia (ie, basic in character turning red litmus blue)" Ammonia will be evolved from an ammonium salt. Thus,

(iii) Solution cf mixture + K₃[Fe(CN)₆] → blue ppt. 

The reaction suggests that the mixture contains a Fe (II) salt. Iron (II) reacts with K₃[Fe(CN)₆] to give a blue ppt. of Prussian blue complex

Red colouration with ammonium thiocyanate suggests that some Fe (III) is also present" It is likely that a part of Fe (II) is oxidised to Fe (III) by air

The above reaction can be explained, when ammonia is liberated on boiling the mixture with potassium hydroxide. The liberated ammonia reacts with an alkaline solution of K₂HgI₄ to give a brown ppt. 

Hence, the mixture contains, Fe²⁺, NH⁺₄,  and Cl^- ions (with an impurity of Fe³⁺ ions). The two salts are : 

FeCl₂ and NH₄Cl.