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

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