`AlF_(3)` is insoluble in anhydrous `HF` but dissolves on addition of `NaF. AlF_(3)` precipitates out of the resulting solution when gaseous `BF_(3)` is bubbled through. Give reasons.

Explanation

Anhydrous `HF` is a covalent compound and is strongly H-bonded. Therefore, it does not furnish `F^(Ө)` ions and `AlF_(3)` remain undissolved. When `NaF` is addedm `F^(Ө)` ions are furnished as `NaF` is an ionic compound. `AlF_(3)` reacts with `F^(Ө)` ions to form ionic compound. `AlF_(3)` reacts with `F^(Ө)` ions to form a complex ion, the sodium salt of which is soluble.
`3NaF + AlF_(3) rarr underset("Soluble complex") (Na_(3)[AlF_(6)]`
Boron, being smaller in size and having higher electronegativity, has greater tendency to form complexes than `Al`. Therefore, when `BF_(3)` is added to the `Na_(3)[AlF_(6)], AlF_(3)` gets precipitated out and boron complex is formed.
`Na_(3)AlF_(6)+ 3BF_(3) rarr 2Na[BF_(4)]+ AlF`.