Question: how many moles of magnesium atom are?
Answer: For example, one atom of magnesium weighs 24.31 amu (atomic mass units). However, one mole of magnesium weighs 24.31 g. (Moles were planned that way!) Since one mole of MgCl2 consists of one mole of magnesium and two moles of chlorine, the mass of one mole of MgCl2 must be the sum of the masses of the elements.