Calculate the number of moles of cl atoms in 3.01×1024 formula units of magnesium chloride, mgcl2.

.01 x 10^24 formula units divided by 6.02 x 10^23 formula units / mol = 5.00 mol of MgCl2 

5.00 mol times 2 moles of Cl atoms / mol of MgCl2 = 10.0 moles of Cl atoms 

The subscript will tell you the moles of a given atom per mole of the formula. For example: 

Al(NO3)3. How many moles of oxygen atoms in one mole of aluminum nitrate? Answer = 9

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