To find the mass of 3.00 moles of magnesium chloride (MgCl2), first record the atomic mass of magnesium (Mg) and chloride (Cl), which are both listed on the periodic table as follows: Mg=24 g/mole Cl=38 g/mole Now, double the Cl mass since there are 2 Cl moles in MgCl2 and then add it to the Mg mass like so: (38 g/mole*2 moles)+24 g/mole=100 g/mole Finally, to calculate the mass of 3.00 moles of MgCl2, convert the combined atomic mass to grams as follows: 3.00 moles * 100 g/mole = 300 g