Question

An inorganic compound `(X)` shows the following reactions :
(a) It is white solid, exists as dimer and fumes in moist air.
(b) It sublimes at `180^@ C` and forms monomer on heating to `400^@ C`.
( c) Its aqueous solution turns blue litmus red and gives a white precipitate with `AgNO_(3)` solution which is soluble in excess of `NH_(4) OH`.
(d) Addition of `NaOH` to the solution of `(A)` gives a white gelatinous precipitate which is, however, soluble on excess of `NaOH`.
Identify the compound `(A)`.

Answer 1

`(A)` is `Al_(2)Cl_(6)`.
(a) `Al_(2)Cl_(6)` is a white solid and exists as dimer.

`Al_(2) Cl_(6)` fumes in moist air due to evolution of `HCl` gas.
`Al_(2) Cl_(6) + 6H_(2) O rarr 2Al(OH)_(3) + 6HCl uarr`.
(b) `Al_(2)Cl_(6(s)) overset(180^@ C)rarr Al_(2) Cl_(6(g)) overset (400^@ C)rarr 2AlCl_(3)`
( c) Aqueous solution of `Al_(@) Cl_(6)` contains hydrated aluminium chloride, `Al_(2)Cl_(6)`, which is ionic in nature.
`Al_(2)Cl_(6)+ 6H_(2) O rarr Al_(2) Cl_(6).6H_(2) O` or `2AlCl_(3).6H_(2)O`
`AlCl_(3(aq)) hArr Al_((aq))^(3+) + 3Cl_((aq))^(Ө)`
Since the above solution is acidic, it turns blue litmus red.
`AlCl_(3) + 3H_(2)O rarr underset("Weak base")(Al(OH)_(3)) + underset("Strong acid")(3HCl)`
On addition of `AgNO_(3)` solution in aqueous solution of aluminium chloride, white precipitate of `AgCl` is formed.
`Cl_((aq))^(Ө) + Ag_((aq))^(oplus) rarr underset(("White ppt.")) (AgCl darr)`
`AgCl` is soluble in excess of `NH_(4)OH` due to the formation of soluble complex, `[Ag(NH_(3))_(2)] Cl`.
`AgCl +2NH_(4)OH rarr underset("Diamminesilver(I)chloride")([Ag(NH_(3))_(2)]Cl + 2 overset(Ө)O H)`
(d) Aqueous solution of `(A) + NaOH rarr underset("Whiteppt. dissolves")underset(darr"Excess "NaOH)underset("gelatinous ppt.")(White)`
`Al^(3+)+NaOH rarr underset("White ppt.") (Al(OH)_(3) darr) + Na^(oplus)`
`Al(OH)_(3) + underset("Excess")(NaOH) rarr underset("Sodium metaluminate") (NaAlO_(2)) + 2H_(2) O`
Due to the formation of `Al(OH)_(3)` initially, white gela-tionous precipitate is obtained, which dissolves on excess of `NaOH` due to the formation of sodium meta-aluminate.