Which equivalence factor set should you use to convert from 4.23 x 1012 atoms of ca to grams of ca? a) (1 mol ca/6.02 x 1023 atoms ca)(40.08 g ca/1 mol ca) b) (1 mol ca/4.23 x 1012 atoms ca)(40.08 g ca/1 mol ca) c) (4.23 x 1012 atoms ca/1 mol ca)(1 mol ca/40.08 g ca) d) 4.23 x 1012 atoms ca/6.02 x 1023 atoms ca)(40.08 g ca)

* 40.08 g Ca

Explanation:
1- One mole of any substance contains Avogadro's number of atoms. Avogadro's number = 6.02 * 10²³ 
To get the number of moles is 4.23 * 10¹² atoms, we will use cross multiplication as follows:
1 mole .................> 6.02 * 10²³ atoms
x moles ................> 4.23 * 10¹²
x = (4.23 * 10¹² atoms Ca) / (6.02 * 10²³ atoms Ca) moles 

2- Number of moles can be calculated as follows:
number of moles = mass / molar mass
We know that the molar mass of Ca = 40.08 g/mol, therefore:
number of moles of Ca = mass / 40.08
This means that:
mass of Ca = number of moles of Ca * 40.08

Substitute with the number of moles (x) calculated in part 1 in the above equation to get the mass as follows:
mass of Ca =  * 40.08 g Ca

Hope this helps :)