Question

Aluminium trifluoride is treated with anhydrous HF and then with NaF. When gaseous BF₃ is passed through the solution obtained, a precipitate X is formed. The formed pricipitate X is 

(a) Na₃ [AlF₆] 

(b) Na[BF₄]

(c) AlF₃ 

(d) H₃ [AlF₆]

Answer 1

Correct option is (b) Na[BF₄]

Anhydrous HF is a covalent compound and is strongly H-bonded. Therefore, it does not give F^– ions and hence AlF₃ does not dissolve in HF. NaF is an ionic compound. It contains F– ions which combine with electron deficient AlF₃ to form the soluble complex.

Boron due to its small size and higher electronegativity has greater tendency to form complexes than Al. Hence, precipitation of AlF₃ takes place when BF₃ is passed through Na₃ [AlF₆] solution.