Ar & Mr (Cambridge O Level Chemistry)

Relative Atomic Mass

• The symbol for the relative atomic mass is A_r
• The relative atomic mass for each element can be found in the Periodic Table along with the atomic number
• The relative atomic mass is shown underneath the atomic symbol and is larger than the atomic number (except for hydrogen where they are the same)
• Atoms are too small to accurately weigh but scientists needed a way to compare the masses of atoms 
• The carbon-12 is used as the standard atom and has a fixed mass of 12 units 
• It is against this atom which the masses of all other atoms are compared 
• Relative atomic mass (A_r) can therefore be defined as:
  • The average mass of the isotopes of an element compared to 1/12^th of the mass of an atom of ¹²C 
• The relative atomic mass of carbon is 12
  • The relative atomic mass of magnesium is 24 which means that magnesium is twice as heavy as carbon 
  • The relative atomic mass of hydrogen is 1 which means it has one twelfth the mass of one carbon-12 atom 

Relative molecular (formula) mass

• The symbol for the relative molecular mass is M_r and it refers to the total mass of the molecule
• To calculate the M_r of a substance, you have to add up the relative atomic masses of all the atoms present in the formula
• Relative formula mass is used when referring to the total mass of an ionic compound

Relative Formula Mass Calculations Table

Reacting masses

• The Law of Conservation of mass tells us that mass cannot be created or destroyed
• In a chemical reaction, the total mass of reactants equals the total mass of the products
• We can use this, along with relative atomic/formula masses to perform calculations to identify the quantities of reactants or products involved in a chemical reaction

• Example:

2Ca + O₂ → 2CaO

• Relative atomic masses: Ca = 40; O = 16
• Using the balanced symbol equation shows that 2 x 40 = 80 units of mass of calcium react with 2 x 16 = 32 units of mass of oxygen to form 2 x (40 + 16) = 112 units of mass of CaO:

2Ca + O₂ → 2CaO

80 + 32   =   112

• The ratio of the mass of calcium and oxygen reacting will always be the same, regardless of the units
  • E.g. 80 g of calcium will react with 32 g of oxygen to form 112 g of calcium oxide
  • Or, 40 tonnes of calcium will react in excess oxygen to form 56 tonnes of calcium oxide