1.4: Understand the terms: (i) ‘relative atomic mass’ based on the 12C scale (ii) ‘relative molecular mass’ and ‘relative formula mass’, including calculating these values from relative atomic masses The term ‘relative formula mass’ should be used for compounds with giant structures. (iii) ‘molar mass’ as the mass per mole of a substance in g mol-1 iv parts per million (ppm), including gases in the atmosphere

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Definitions:

  • Relative atomic mass: The weighted average mass of all the isotopes of an element compared to 1/12th of the mass of a carbon-12 atom. This is the value shown on your periodic table.
  • Relative molecular mass: The weighted average of the masses of the molecules of a compound relative to 1/12th of the mass of a carbon-12 atom.
  • Relative formula mass: This is the weighted average of the masses of the formula units of a substance relative to 1/12th of the mass of a carbon-12 atom. (This is only used in macromolecules such a graphite or diamond).
  • Molar mass: This is the mass of a given substance (element or compound) divided by the amount of substance. Measured in g/mol.
  • Parts per million (ppm): This is the concentration by volume of one part of gas, or by weight of one part of a liquid or solid, per million parts of air or liquid. It is just like a percentage (%), but instead of being out of 100 it is out of 1,000,000.

Examples:

        1. What is the relative molecular mass of the following compounds:

  • H₂O:
    • First, find the Mᵣ of the different elements found in the compound:
      • Mᵣ of H = 1, Mᵣ of O = 16.
    • Next, multiply the Mᵣ of each element by the number of atoms present in the compound and add the total up:
      • 2 x (1) + 1 x (16) = 18.
    • So, the relative molecular mass of H₂O = 18
  • CaCO₃:
    • First, find the Mᵣ of the different elements found in the compound:
      • Mᵣ of Ca = 40.1, Mᵣ of C = 12, Mᵣ of O = 16
    • Next, multiply the Mᵣ of each element by the number of atoms present in the compound and add the total up:
        • 1 x (40.1) + 1 x (12) + 3 x (16) = 100.1
      • So, the relative molecular mass of CaCO₃ = 100.1


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