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

Key Concepts

  • Relative molecular mass is also known as molecular weight, relative molar mass, molar weight, formula mass and formula weight.

  • Relative molecular mass is usually given the symbol Mr
        Other symbols commonly used are MM, MW, FM, FW*.

  • Relative molecular mass of a compound is defined as the mass of a formula unit of the compound relative to the mass of a carbon atom taken as exactly 12.

  • In practice, the relative molecular mass, Mr, of a compound is the sum of the relative atomic masses (atomic weights) of the atomic species as given in the chemical formula.

  • Relative molecular mass is a dimensionless quantity, it has no units #.

Relative Molecular Mass Calculations

Example 1

Calculate the relative molecular mass (Mr) of the compound carbon monoxide, CO
  1. Determine the number of atoms of each element present in the chemical formula:
        CO is composed of one atom of C (carbon) and one atom of O (oxygen)

  2. Use the Periodic Table to find the relative atomic mass (atomic weight) for each element present:
        relative atomic mass of C (carbon) = 12.0
        relative atomic mass of of O (oxygen) = 16.0

  3. Calculate the relative molecular mass
    Mr = sum of the atomic masses present in the formula
    Mr(CO) = (1 x relative atomic mass of carbon) + (1 x relative atomic mass of oxygen)
    Mr(CO) = (1 x 12.0) + (1 x 16.0) = 28.0

Example 2

Calculate the relative molecular mass (Mr) of the compound carbon dioxide, CO2
  1. Determine the number of atoms of each element present in the chemical formula:
        carbon dioxide is composed of one atom of C (carbon) and two atoms of O (oxygen)

  2. Use the Periodic Table to find the relative atomic mass (atomic weight) for each element present:
        relative atomic mass of C (carbon) = 12.0
        relative atomic mass of O (oxygen) = 16.0

  3. Calculate the relative molecular mass
    Mr = sum of the atomic masses present in the formula
        Mr(CO2) = relative atomic mass carbon + (2 x relative atomic mass oxygen)
        Mr(CO2) = 12.0 + (2 x 16.0) = 12.0 + 32.0 = 44.0

Example 3

Calculate the relative molecular mass (Mr) of the compound calcium hydroxide, Ca(OH)2
  1. Determine the number of atoms of each element present in the chemical formula:
        The formula for calcium hydroxide contains one Ca atom (actually an ion) and two hydroxide ions.
        Each hydroxide ion is composed of one hydrogen atom and one oxygen atom, so calcium hydroxide contains 2 oxygen atoms and 2 hydrogen atoms

  2. Use the Periodic Table to find the relative atomic mass (atomic weight) for each element present:
        relative atomic mass calcium = 40.1
        relative atomic mass hydrogen = 1.0
        relative atomic mass of oxygen = 16.0

  3. Calculate the relative molecular mass
    Mr = sum of the atomic masses present in the formula
        Mr(Ca(OH)2) = (1 x relative atomic mass Ca) + (2 x relative atomic mass O) + (2 x relative atomic mass H)

        Mr(Ca(OH)2) = 40.1 + (2 x 16.0) + (2 x 1.0) = 40.1 + 32.0 + 2.0 = 74.1

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

        Mr(Ca(OH)2) = 40.1 + [2 x (16.0 + 1.0)] = 40.08 + [2 x 17.0]= 40.1 + 34.0 = 74.1

* recommended IUPAC symbol for relative molecular mass (relative molar mass) is Mr.
M is the IUPAC symbol recommended for molar mass

#Molar mass, M, does have units. The SI units for molar mass are kg mol-1


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