Two moles of an unknown substance were shown to have a mass of 240g. What molecular mass does the substance have?

Molecular mass or molecular weight is the mass of a molecule. It is calculated as the sum of the atomic weights of each constituent element multiplied by the number of atoms of that element in the molecular formula. So if there are 2 moles, then it is multiplied to 240 g, which will result to 480 g - - the molecular mass of the substance.