Relative Molecular Mass and Relative Formula Mass
Lajoy Tucker
Teacher
Contents
Relative Atomic Mass (Aᵣ)
The relative atomic mass of an element is the weighted average mass of an atom of an element, compared to 1/12th the mass of a carbon-12 atom.
Details on calculating relative atomic mass can be found in the Isotopes and Relative Atomic Mass note
Relative Atomic Masses of all elements can be found on your Periodic Table and for AQA A-level are quoted to one decimal place
Relative Molecular Mass (Mᵣ)
Relative molecular masses are calculated as the sum of the relative atomic masses of the atoms in the molecule
Example 1: Water ()
The molecular formula of water is
The relative atomic masses of hydrogen and oxygen are and , respectively
of ()
of ()
The relative molecular mass of water is
No answer provided.
Example 2: Trichloromethane ()
The molecular formula of Trichloromethane is .
The relative atomic masses of carbon, hydrogen, and chlorine are , , and , respectively.
of
of
The relative molecular mass of is .
No answer provided.
Relative Formula Mass
The relative formula mass is essentially the same as the relative molecular mass but applies to giant structures e.g. ionic compounds, where species do not exist as discrete units.
Example: Sodium Chloride ()
The formula of sodium chloride is .
The relative atomic masses of sodium and chlorine are and , respectively.
of
The relative formula mass of sodium chloride is .
No answer provided.
Example: Copper(II)Sulfate Crystals, ()
is an example of a dot formula.
The represents water of crystallisation in a hydrated ionic compound. These water molecules are a key part of the lattice structure and must be included when calculating .
of
of
No answer provided.
Tip!
Students usually remember to multiply the 2 hydrogens by 5 but forget to multiply the oxygen by 5. If you add the water up as a whole, you limit your chances of making this mistake.
No answer provided.
Practice Questions
Calculate the relative formula/relative molecular masses of the following species.
a)
b)
c)
Answer
a)
b)
c)
Key points to remember:
1. Relative atomic mass accounts for isotopes' masses and abundances
2. Relative molecular mass is used for covalent molecules and calculated by summing the atomic masses of the atoms in the molecule
3. Relative formula mass applies to ionic compounds, calculated similarly to relative molecular mass but only for the empirical formula.
No answer provided.