Aqueous solution contains either aluminium sulphate or zinc sulphate. which aqueous reagent can be used to confirm which salt is present?
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The addition of H2SO4 only brings more SO4^2- and HSO4- ions to the solution, which does nothing. The addition of Ba2+ ions will produced a precipitate in both solutions of BaSO4. That tells us nothing, as well.
Sodium hydroxide CANNOT be be used to distinguish Al2(SO4)3(aq) from ZnSO4(aq). The addition of OH- to the Al2(SO4)3(aq) will produce a gelatinous precipitate, but as more OH- is added, the precipitate will dissolve, forming Al(OH)4^-. When OH- ions are added to ZnSO4(aq) a precipitate of Zn(OH)2 will form, but as additional OH- is added the solid Zn(OH)2 dissolves forming Zn(OH)4^2- ions. You will get the same effect for both salts.
You can distinguish between Al and Zn sulfates using ammonia (NH3). As with NaOH solution, you get hydroxide precipitates of Al(OH)3 and Zn(OH)2 since a solution of NH3 contains a small amount of OH- ion. With the addition of more NH3 solution only the Zn(OH)2 will dissolve. It is not because it goes to the soluble hydroxide form, since there are not enough OH- present for that, but Zn(OH)2 dissolves to form [Zn(NH3)4]2+ and Al(OH)3 does not.
Sodium hydroxide CANNOT be be used to distinguish Al2(SO4)3(aq) from ZnSO4(aq). The addition of OH- to the Al2(SO4)3(aq) will produce a gelatinous precipitate, but as more OH- is added, the precipitate will dissolve, forming Al(OH)4^-. When OH- ions are added to ZnSO4(aq) a precipitate of Zn(OH)2 will form, but as additional OH- is added the solid Zn(OH)2 dissolves forming Zn(OH)4^2- ions. You will get the same effect for both salts.
You can distinguish between Al and Zn sulfates using ammonia (NH3). As with NaOH solution, you get hydroxide precipitates of Al(OH)3 and Zn(OH)2 since a solution of NH3 contains a small amount of OH- ion. With the addition of more NH3 solution only the Zn(OH)2 will dissolve. It is not because it goes to the soluble hydroxide form, since there are not enough OH- present for that, but Zn(OH)2 dissolves to form [Zn(NH3)4]2+ and Al(OH)3 does not.
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