Ar & Mr (Cambridge O Level Chemistry)

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Relative Masses

Relative Atomic Mass

  • The symbol for the relative atomic mass is Ar
  • 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 (Ar) can therefore be defined as:
    • The average mass of the isotopes of an element compared to 1/12th of the mass of an atom of 12
  • 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 Mr and it refers to the total mass of the molecule
  • To calculate the Mr 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

Substance, atoms present table, IGCSE & GCSE Chemistry revision notes

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

Worked example

Calculate the mass of carbon dioxide produced when 32 g of methane, CH4, reacts completely in excess oxygen:

CH4 + 2O2 → CO2 + 2H2O

Relative atomic masses, Ar: H = 1; C = 12; O = 16

Answer

  • In terms of relative mass the equation is:

                CH4      +       2O2        →         CO2     +        2H2O

              12 + (4 x 1) + 2 x (2 x 16) → 12 + (2 x 16) + 2 x (2 x 1 + 16)

               16       +          64        →         44        +         36

  • So 16 g of methane would react in excess oxygen to form 44 g of carbon dioxide
  • Therefore, 32 g of methane would produce 44 x 2 = 88 g of carbon dioxide

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Caroline

Author: Caroline

Expertise: Physics Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.