# 11.16.    Aluminium trifluoride is insoluble in anhydrous $H\! F$ but dissolves on addition of $N\! aF$. Aluminium trifluoride precipitates out of the resulting solution when gaseous  $B\! F_{3}$  is bubbled through. Give reasons.

$H\! F$ is a covalent compound and has a very strong intermolecular hydrogen bonding. So, it does not provide any ions, as a result, $AlF_{3}$ does not dissolve int it. $N\! aF$ is an ionic compound and it can give $F^{-}$ ions. Thus $AlF_{3}$ can dissolve in it and form $Na_{3}[AlF_{6}]$
$AlF_{3}+3NaF\rightarrow Na_{3}[AlF_{6}]$ (sodium hexafluoroaluminate(III))
When $B\! F_{3}$ is added to the solution, it can replace aluminium and formed a complex of itself and $AlF_{3}$ can precipitate out.
$Na_{3}[AlF_{6}]+BF_{3}\rightarrow 3Na[BF_{4}]+AlF_{3}$