Amphoterism Amphoteric oxides, such as aluminium oxide, are soluble both in strongly acidic and in strongly base solutions : In acid:`Al_2O_3(s)+6H_3O^(+)(aq)hArr2Al^(3+)(aq)+9H_2O(l)` In base:`Al(OH)_3(s)+OH^(-)(aq)hArr Al(OH)_4^(-)(aq)` Dissolution of `Al(OH)_3` in excess base is just a special case of the effect of complex-ion formation on solubility.`Al(OH)_3` dissolves because excess `OH^(-)` ions convert it to the soluble complex ion `Al(OH)_4^(-)`(aluminate ion)The effect of pH on the solubility of `Al(OH)_3` is shown in figure. Other examples of amphoteric hydroxides include `Zn(OH)_2,Cr(OH)_3,Sn(OH)_2 and Pb(OH)_2`, which react with excess `OH^(-)` ions to form the soluble complex ion `Zn(OH)_4^(2-)`(zincate ion),`Cr(OH)_4^(-)`(chromite ion),`Sn(OH)_3^(-)` , `Fe(OH)_2 and Fe(OH)_3` , dissolve in strong acid but not in strong base. `Zn(OH)_2` is a amphoteric hydroxide and is involved in the following two equilbria in aqueous solutions `Zn(OH)_2(s)hArrZn^(2+)(aq)+2OH^(-)(aq),K_(sp)=1.2xx10^(-17)` `Zn(OH)_2(s)+2OH^(-)(aq)hArr[Zn(OH)_4]^(2-)(aq),K_(@)=0.12` At what pH the solubility of `Zn(OH)_2` be minimum ?A. 4B. 10C. 6D. 8

Amphoterism Amphoteric oxides, such as aluminium oxide, are soluble bo

1.	Amphoterism Amphoteric oxides, such as aluminium oxide, are soluble both in strongly acidic and in strongly base solutions : In acid:`Al_2O_3(s)+6H_3O^(+)(aq)hArr2Al^(3+)(aq)+9H_2O(l)` In base:`Al(OH)_3(s)+OH^(-)(aq)hArr Al(OH)_4^(-)(aq)` Dissolution of `Al(OH)_3` in excess base is just a special case of the effect of complex-ion formation on solubility.`Al(OH)_3` dissolves because excess `OH^(-)` ions convert it to the soluble complex ion `Al(OH)_4^(-)`(aluminate ion)The effect of pH on the solubility of `Al(OH)_3` is shown in figure. Other examples of amphoteric hydroxides include `Zn(OH)_2,Cr(OH)_3,Sn(OH)_2 and Pb(OH)_2`, which react with excess `OH^(-)` ions to form the soluble complex ion `Zn(OH)_4^(2-)`(zincate ion),`Cr(OH)_4^(-)`(chromite ion),`Sn(OH)_3^(-)` , `Fe(OH)_2 and Fe(OH)_3` , dissolve in strong acid but not in strong base. `Zn(OH)_2` is a amphoteric hydroxide and is involved in the following two equilbria in aqueous solutions `Zn(OH)_2(s)hArrZn^(2+)(aq)+2OH^(-)(aq),K_(sp)=1.2xx10^(-17)` `Zn(OH)_2(s)+2OH^(-)(aq)hArr[Zn(OH)_4]^(2-)(aq),K_(@)=0.12` At what pH the solubility of `Zn(OH)_2` be minimum ?A. 4B. 10C. 6D. 8
Answer» Correct Answer - B Let solubility of `Zn(OH)_2`=s, some of will go in `Zn^(2+)` form and some in complex `[Zn(OH)_4]^(2-)`, of `[OH^(Theta)]=10^(-x)M`, then `S_1(10^(-14)/10^(-x))^2=1.2xx10^(-17) `...(i) `S_2/(10^(-14)/10^(-x))^2=0.12` [`S_1+S_2`=total solubility] So , `S_1S_2=1.2xx1.2 10^(-18)` Now we want `S=S_1+S_2` to be minimum we will have `S_1=S_2` So, `S_1=S_2=1.2xx10^(-9)M` Hence from `1^(st)` equation we get `(10^(-14)/10^(-x))^2=(1.2xx10^(-17)) /(1.2xx10^(-3))=10^(-10)` Hence, x=10

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