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

the number of valence electrons for each molecule or ion is shown beneath the structure. remember hydrogen will not have more than two electrons. this hydrogen is part of a covalent bond (sharing two electrons). a double bond here would cause hydrogen to share four electrons with phosphorus.

the right answer is c i just did the question.