Reacting masses and chemical equations
● The relative atomic mass Ar, for an element is the average mass of its naturally occurring
isotopes relative to the mass of a carbon-12 atom, the atomic number of an element.
● 2 laws of chemistry
○ Elements always react in the same ratio, to form a given compound
○ The total mass does not change, during a chemical reaction. So total mass of reactants =
total mass of products
● Percentage composition
○ Write down the formula of the compound
○ Using A, values, work out its molecular or formula mass (MR)
○ Write the mass/MR
○ Multiply the fraction by 100 to give the percentage
● Percentage purity
○ Purity of a substance = mass of a pure substance/total mass x 100%
