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Aluminum trifluoride is insoluble in anhydrous HF but dissolves when NaF is added. It precipitates out of the resulting solution when gaseous BF3(boron trifluoride) is bubbled through. Give reasons.

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Hydrogen fluoride is a covalent compound and has a very strong intermolecular hydrogen-bonding. Thus, it does not provide ions and aluminum fluoride does not dissolve in it. Sodium fluoride is an ionic compound and when it is added to the mixture, Alf dissolves. This is because of the availability of free F . The reaction involved in the process is:

AlF3+3NaF→Na3[AlF6]

Aluminum fluoride gets precipitated out of the solution when boron trifluoride is added to the solution. This happens because the tendency of boron to form complexes is much more than that of aluminum. Therefore, when boron trifluoride is added to the solution, B replaces Al from the complexes according to the following reaction:

Na3[AlF6]+3BF3→3Na[BF4]+AlF3

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