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Aluminium trifluoride is insoluble in an...

Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous `BF_3` is bubbled through. Give reasons.

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Anhydrous HF being a strongly H -bonded covalent compound does not give `F^(-)` ions for complexing with `AIF_3` and so `AIF_3` does not dissolve in HF. In the presence of the ionic compound NaF, `F^(-)` -ions combine with `AIF_3` to form soluble complex.
`3NaF + AIF_3 to Na_3[AIF_6]underset(("Soluble complex"))("Sodiumhexafluoroaluminate (III)")`
Due to small size and higher electronegativity, boron has much higher tendency to form complex than aluminium. Because of this, when gaseous `BF_3` is bubbled through this solution, aluminium trifluoride precipitates out.
`Na_3[AIF_6]+3BF_3tounderset("Sodium tetrafluoroborate ")(3Na[BF_4]"(soluble complex)")+AIF_3`
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