3- a) write the formula unit of calcium fluoride. b) A certain mass m of calcium fluoride contains 1.2*10^23 anions. Deduce the mass (m). NA= 6.02*10^23

a) CaF₂.

b) 7.81g of CaF₂ are present.

Explanation:

a) The calcium ion has as charge Ca²⁺ and fluoride ion is F⁻, that means formula unit is:

CaF₂

b) 1 molecule of CaF₂ contains 2 anions, F⁻. Thus, the moles of CaF₂ is:

1.2x10²³ anions * (1 moleculeCaF₂ / 2 anions) = 6x10²² molecules of CaF₂

6.022x10²³ molecules = 1mol:

6x10²² molecules of CaF₂ * (1mol / 6.022x10²³molecules) = 0.10 moles CaF₂.

1 mole of CaF₂ has a mass of 78.07g:

0.10 moles CaF₂ * (78.07g / mol) =

7.81g of CaF₂ are present

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