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

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Hint: Cryolite consists of sodium, fluorine and aluminum. In cryolite, the oxidation state of sodium is $+3$ , fluorine is $-6$ and aluminum is $+3$ . Now with the help of this you can easily identify the chemical formula of the cryolite.

Complete answer:
Cryolite is a very rare mineral and is found at the Ivittuut on the west coast of Greenland and it is one of the largest deposits of cryolite. Other small deposits of the mineral cryolite are found in Montreal, Canada, Russia etc.
Cryolite is reddish to greyish-black colored crystals and is readily soluble in water and becomes completely invisible in it.
The chemical formula of cryolite is $N{{a}_{3}}Al{{F}_{6}}$ and therefore, commonly, cryolite is also known as sodium hexafluoro aluminate .
The cryolite is mainly used in the process of the electrolysis. In the process of electrolysis, instead of using the aluminum oxide alone, the mixture of aluminum oxide and cryolite is used. It is so because mixture of cryolite and aluminum oxide has very low melting point and thus, a very low amount of energy is required for the process of electrolysis and thus, its cost is also reduced.
So, thus the formula of cryolite is $N{{a}_{3}}Al{{F}_{6}}$.

Hence, option (b) is correct.

Note:
The molten cryolite is used as a solvent and it helps in increasing the conductivity of the solution. Moreover, cryolite is also used as the bio-control agent i.e. as insecticide and pesticide.