A colourless mixture of two salts `(A)` and `(B)` (excess) is soluble in `H_(2) O. (A)` turns blue litmus red and `(B)` turns red litmus blue. `(A)` gives white precipitate with `(B)`, which dissolves in excess of `(B)` forming `( C).(A)` when placed in atmosphere gives fumes and can form dimer. `(A)` gives white precipitate with `NH_(4) Cl` and `NH_(4)OH` which is soluble in `(B)`. Identify `(A), (B)` and `(C)`. Explain reactions.

Explanation

`(A)` gives white pptl with `NH_(4) Cl` and `NH_(4) OH`. Hence, `(A)` is `Al^(3+)` and , white precipitate is of `Al(OH)_(3)`.
`Al(OH)_(3)` is soluble in `(B)`, therefore, `(B)` is `NaOH` and `( C)` is `NaAlO_(2)` (sodium meta-aluminate).
`(A)` forms dimer which indecates `(A)` is electron-deficient compound that is possible in case of `AlCl_(3)`.
`(A)` is thus `AlCl_(3)` (acidic due to hydrolysis, turns blue litmus red).
`(B)` is `NaOH` (alkaline, turns red litmus blue).
`underset((A))(AlCl_(3)) + underset((B))(3NaOH) rarr underset("White ppt.")(Al(OH)_(3))+ 3NaCl`
`Al(OH)_(3) + underset(("Excess"))(NaOH) rarr underset(underset((C))("Soluble"))(NaAlO_(2)) + H_(2) O`
`AlCl_(3) +3H_(2)P rarrAl(OH)_(3) + 2HCl uarr(Fumes)`
`AlCl_(3) + 3NH_(4) OH overset(Na_(4)Cl)rarr Al(OH)_(3) + 3NH_(4)Cl`
`2AlCl_(3) rarr Al_(2)Cl_(6)`.
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