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 :)

the catch is that the top numbers are atomic masses the bottom numbers are the numbers of protons and electrons in an uncharged atom. they are the number of that atoms as you would find them on the periodic table. further, the numbers on the left are used to determine the missing numbers on the right

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer

Maybe you should get some from your teacher and learn more so you can get your grades up and maybe study a bit more