Give reason for the following : (i) Zinc is not precipitated as Zn(OH)_(2)on adding NH_(4)OH to a zinc salt solution containing NH_(4)Cl. (ii) BaSO_(4) precipitate is washed with water containing a small amount of H_(2)SO_(4) in gravimetric analysis. (iii) CO_(2) is more soluble in aqueous NaOH solution than in water. (iv) A brown precipitate in a bottle containing aqueous FeCl_(3) solution appears pm standing.

Give reason for the following : (i) Zinc is not precipitated as Zn(OH

1.	Give reason for the following : (i) Zinc is not precipitated as Zn(OH)_(2)on adding NH_(4)OH to a zinc salt solution containing NH_(4)Cl. (ii) BaSO_(4) precipitate is washed with water containing a small amount of H_(2)SO_(4) in gravimetric analysis. (iii) CO_(2) is more soluble in aqueous NaOH solution than in water. (iv) A brown precipitate in a bottle containing aqueous FeCl_(3) solution appears pm standing.
Answer» Solution :(i) `NH_(4)Cl` suppresses the ionization of `NH_(4)OH` due to common ion effect. Hence, concentration of `OH^(-)` decreases such that the ionic product `[Zn^(2+)][OH^(-)]^(2)` doesnot exceed solubility product. Hence, `Zn(OH)_(2)` is not precipitated. (ii) `BaSO_(4)` dissociates as : `BaSO_(4)(s)overset(aq)hArrBa^(2+)(aq)+SO_(4)^(2-) (aq)`. Addition of `H_(2)SO_(4)` gives common `SO_(4)^(2-)` ions which suppresses the dissociation of `BaSO_(4)` and thus helps in the complete precipitation. (iii) In WATER, `CO_(2)` dissolves to formcarbonic acid `(H_(2)CO_(3))`: `CO_(2)(g)+H_(2)CO_(3)(aq)` As the reaction is reversible, the solubility is low. In aqueous `NaOH, H_(2)CO_(3)` reacts with sodium hydroxide as FOLLOWS : `2NaOH(aq)+H_(2)CO_(3) rarr Na_(2)CO_(3)(aq)+H_(2)O(l)` As a result, the equillibrium involving dissolution of `CO_(2)` in `H_(2)O` shifts FORWARD , i.e., solubility increases. (iv) `FeCl_(3)` undergoes hydrolysis forming a brown precipitate of ferric hydroxide `{:(FeCl_(3)+3H_(2)O,rarr,Fe(OH)_(3)+ 3HCl,,),(,,"Brown ppt.",,):}`