1. 3.1Relative Atomic Mass Relative Mass Law of Constant Composition Relative Masses of Atoms Cannizzaro’s Paper

2. Relative Mass The mass ratio of any equal number of items equals the average mass ratio of those individual items. e.g. If 100 staples weighs twice as much as 100 rice grains then the average mass of a staple is double the average mass of a rice grain. text page 108 3.1Relative Atomic Mass

3. Law of Constant Composition text pages 109-110 This is because the mass ratios in which different elements combine are the mass ratios of their individual atoms or a simple multiple thereof. The law of constant composition states that all samples of a given compound have the same mass ratio of their constituent elements, e.g. there are 8 g of oxygen for every 1 g of hydrogen in every sample of water. 3.1Relative Atomic Mass

4. Relative Masses of Atoms Hydrogen has the least massive atoms of all the elements so its atoms were originally assigned an atomic mass of 1 u (atomic mass unit) and the mass of all the other types of atoms were expressed relative to this. You need to be comparing the masses of an equal number of atoms for their mass ratio to represent the mass ratio of the individual atoms, e.g. if you do a mass analysis of BaCl 2 then you need to double the mass of the Ba to be comparing the masses of an equal number of atoms. text pages 110-111 3.1Relative Atomic Mass

5. Cannizzaro’s Paper text page 111 The ‘composition analysis’ technique for determining relative atomic mass only works if you know the formula of the compound being analyzed. Stanislao Cannizzaro showed how other techniques could be used for determining atomic masses and these atomic masses were, in turn, used to determine which compound formulas were correct. 3.1Relative Atomic Mass