# OCR AS Chemistry Salters B - Amount of Substance

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• Amount of Substance
• 6.02 x 10^23
• tells you the number of particles in 1 mole of a substance
• Relative Atomic Mass (Ar)
• tells you the number of times an atom of an element is heavier than one twelth of Carbon-12
• it is the weighted average of the relative isotopic masses for an element (therefore are not always whole numbers)
• is the average mass of 100 atoms of the same element
• Relative Formula Mass (Mr)
• the sum of the Relative Atomic Masses for each atom in a formula
• Empirical Formula
• tells you the simplest ratio of each element in a compound
• 1- divide the mass of each element by its relative atomic mass
• 2- divide these answers by the smallest answer you obtained in step 1
• 3- this will give you a 1:? ratio
• Molecular Formula
• tells you the actual number of atoms of each element in a molecule