Molecular Mass     (Molecular Weight, Formula Mass, Formula Weight)

Molecular Mass (Molecular Weight)

• In theory, the relative molecular mass or molecular weight of a compound is the mass of a molecule of the compound relative to the mass of a carbon atom taken as exactly 12.

• In practice, the molecular mass, MM, (molecular weight, MW) of a compound is the sum of the atomic masses (atomic weights) of the atomic species as given in the molecular formula.

• In theory we can only refer to the Molecular Mass or Molecular Weight of a covalent compound since only covalent compounds are composed of molecules.

Formula Mass (Formula Weight)

• The relative formula mass, FM, (formula weight, FW) of a compound is the sum of the atomic masses (atomic weights) of the atomic species as given in the formula of the compound.

• Formula Mass (Formula Weight) is a more general term that can be applied to compounds that are not composed of molecules, such as ionic compounds.

• In practice, the terms, molecular mass, molecular weight, formula mass and formula weight are used interchangeably by Chemists.

Molecular Mass Calculations

Calculate the Molecular Mass (MM) of the compound carbon monoxide, CO

• The formula for carbon monoxide is composed of one atom of carbon and one atom of oxygen

• Atomic mass carbon = 12.01 (from the Periodic Table)
  Atomic mass of oxygen = 16.00 (from the Periodic Table)

• Molecular Mass (MM) for cabon monoxide
      = atomic mass carbon + atomic mass oxygen

  Molecular mass (MM) = 12.01 + 16.00
      = 28.01 g/mole

Calculate the Molecular Mass (MM) of the compound carbon dioxide, CO₂

• The formula for carbon dioxide is composed of one atom of carbon and two atoms of oxygen

• Atomic mass carbon = 12.01 (from the Periodic Table)
  Atomic mass of oxygen = 16.00 (from the Periodic Table)

• Molecular Mass (MM) for carbon dioxide
      = atomic mass carbon + (2 x atomic mass oxygen)

  Molecular Mass (MM) = 12.01 + (2 x 16.00)
      = 12.01 + 32.00
      = 44.01g/mole

Calculate the Molecular Mass (MM) of the compound water, H₂O

• The formula for water is composed of two hydrogen atoms and one oxygen atom

• Atomic mass hydrogen = 1.008 (from the Periodic Table)
  Atomic mass of oxygen = 16.00 (from the Periodic Table)

• Molecular Mass (MM) for water
      = (2 x atomic mass hydrogen) + atomic mass oxygen

  Molecular Mass (MM) = (2 x 1.008) + 16.00
      = 2.016 + 16.00
      = 18.016g/mole

Calculate the Molecular Mass (MM) of the compound calcium hydroxide, Ca(OH)₂

• The formula for calcium hydroxide is composed of one calcium "atom" (actually an ion) and two hydroxide ions. Each hydroxide ion is composed of one hydrogen "atom" (actually an ion) and one oxygen "atom" (also an ion)

• Atomic mass calcium = 40.08 (from the Periodic Table)
  Atomic mass hydrogen = 1.008 (from the Periodic Table)
  Atomic mass of oxygen = 16.00 (from the Periodic Table)

• Molecular Mass (MM) for calcium hydroxide
      = atomic mass calcium + (2 x atomic mass oxygen) + (2 x atomic mass hydrogen)

  Molecular Mass (MM) = 40.08 + (2 x 16.00) + (2 x 1.008)
      = 40.08 + 32.00 + 2.016
      = 74.096g/mole

  Alternatively, Moelcular Mass = atomic mass of calcium + (2 x molecular mass of hydroxide ions)

  Molecular Mass (MM) = 40.08 + [2 x (16.00 + 1.008)]
      = 40.08 + [2 x 17.008]
      = 40.08 + 34.016
      = 74.096g/mole

Calculate the Molecular Mass (MM) of the compound ammonium sulfate, (NH₄)₂SO₄

• The formula of ammonium sulfate is composed of two atoms of nitrogen, eight atoms of hydrogen, one atom of sulfur and four atoms of oxygen

• Atomic mass nitrogen = 14.01 (from the Periodic Table)
  Atomic mass hydrogen = 1.008 (from the Periodic Table)
  Atomic mass of sulfur = 32.06 (from the Periodic Table)
  Atomic mass of oxygen = 16.00 (from the Periodic Table)

• Molecular Mass (MM) for ammonium sulfate
      = (2 x atomic mass nitrogen) + (8 x atomic mass hydrogen) + atomic mass sulfur + (4 x atomic mass oxygen)

  Molecular Mass (MM) = (2 x 14.01) + (8 x 1.008) + 32.06 + (4 x 16.00)
      = 28.02 + 8.064 + 32.06 + 64.00
      = 132.144g/mole

  Alternatively, Molecular Mass (MM) = [2 x molecular mass ammonium ions (NH₄)] + atomic mass sulfur + (4 x atomic mass oxygen)

  Molecular Mass (MM) = {2 x [14.01 + (4 x 1.008)]} + 32.06 + (4 x 16.00)
      = {2 x [14.01 + 4.032]} + 32.06 + 64.00
      = {2 x 18.042} + 32.06 + 64.00
      = 36.084 + 32.06 + 64.00
      = 132.144g/mol

Calculate the Molecular Mass (MM) of the compound ethanoic acid (acetic acid), CH₃COOH

• The formula of ethanoic acid is composed of two carbon atoms, four hydrogen atoms and two oxygen atoms

• Atomic mass carbon = 12.01 (from the Periodic Table)
  Atomic mass hydrogen = 1.008 (from the Periodic Table)
  Atomic mass of oxygen = 16.00 (from the Periodic Table)

• Molecular Mass (MM) for ethanoic acid
      = (2 x atomic mass carbon) + (4 x atomic mass hydrogen) + (2 x atomic mass oxygen)

  Molecular Mass (MM) = (2 x 12.01) + (4 x 1.008) + (2 x 16.00)
      = 24.02 + 4.032 + 32.00
      = 60.052g/mol

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