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
Calculate the Molecular Mass (MM) of the compound carbon dioxide, CO2
Calculate the Molecular Mass (MM) of the compound water, H2O
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
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