How many moles of MgCl2 are there in 324g of the compound?

So you use the molar mass, 95.211 g/mol to find the number of moles.

324 g MgCl2 • (1 mol / 95.211 g) = approx 3.403 moles

When doing conversions, set up fractions so that the units you want gone cancel diagonally.