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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 ? |
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Answer» 4 `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` Hencefrom `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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