Question

How many formula units make up 20.8g of magnesium chloride (MgCl2)?

Answer 1

We need to find the number of MgCl2 "molecules" in the given mass. Since MgCl2 forms a crystal lattice and not individual molecules, we instead talk about "formula units", but it's the same idea.

So all you need to do is divide the given mass by the molar mass of the formula unit (MgCl2) to get the moles. Then multiply this number by Avogadro's number to get the total number of formula units!