Question

What is the formula of cryolite?
(a). $\text{N}{{\text{a}}_{3}}\text{Al}{{\text{F}}_{10}}$
(b). $\text{N}{{\text{a}}_{3}}\text{Al}{{\text{F}}_{6}}$
(c). $\text{N}{{\text{a}}_{3}}\text{Al}{{\text{F}}_{8}}$
(d). $\text{N}{{\text{a}}_{3}}\text{Al}{{\text{F}}_{7}}$\[\]

Answer 1

- Hint: It is the combination of sodium and aluminium inorganic salt whose molecular weight is 209.04gm. It is synthesized by fusion of sodium fluoride and aluminium fluoride as an electrolyte in the reduction of alumina to aluminium metal. It occurs in nature as the mineral cryolite.

Complete step-by-step solution -

Cryolite is trisodium hexafluoroaluminate ($\text{N}{{\text{a}}_{3}}\text{Al}{{\text{F}}_{6}}$). It is an inorganic sodium salt and perfluorometallate salt. It contains hexafluoroaluminate.
We should know about the physical and chemical properties of the compound as well. It is a colourless to white halide mineral, sodium aluminium fluoride. It is used as a solvent for bauxite in the electrolytic production of aluminium and has various other metallurgical applications. It is also used in the glass and enamel industries, in the manufacture of insecticides and in bonded abrasives as the filler.
One of the most important roles of cryolite is in aluminium extraction by the Hall-Heroult process. In the metallurgy of aluminium, the metal is to be isolated from alumina ($\text{A}{{\text{l}}_{2}}{{\text{O}}_{3}}$) by carrying out its electrolytic reduction. The melting point of alumina is very high (2323K). It is therefore mixed with cryolite which lowers its melting point to 1173K.
Moreover, cryolite also increases the electrical conductivity of alumina.
Therefore, the correct option is (b) $\text{N}{{\text{a}}_{3}}\text{Al}{{\text{F}}_{6}}$

Note: We should know that cryolite comes under the category of halide mineral. Halide mineral is any of a group of naturally occurring inorganic compounds that are salts of the halogen acids. (e.g-hydrochloric acid). Such compounds,with the notable exceptions of halite (rock salt), sylvite and fluorite, are rare and of very local occurrence.