Question

How many formula units make up 24.2g of magnesium chloride (MgCl2)?

Answer 1

You're finding the number of molecules that make up that mass. So first, you wanna find the molecular mass (look at periodic table) of MgCl2 first, then divide mass (24.2g) by it to find moles.

moles=mass/Molecular mass

then multiply the moles by avogadros number.