MASS = MR x MOLES
CONCENTRATION = MOLES / VOLUME ( IN DM3)
Each mole contains avogadros constant of particles in it. The amount of grams needed to equal avogadro's constant is, helpfully enough, the atomic mass of the element, in grams. Hence 12g of Carbon-12 is one mole.
Empirical formula is the simplest whole number ratio of elements in a compound. For example, molecular formula C6H12Cl2 simplifies down to C3H6Cl.
EDIT: Haha, awh, you're doing GCSE. That's cute. Fml. :')