Relative Molecular Mass (Mr)
Chemistry: Cambridge International GCSE Supplement, topic 3.2.2
Understanding Relative Molecular Mass (Mr)
Relative Molecular Mass (Mr) is the sum of relative atomic masses in a molecule. For ionic compounds, it's Relative Formula Mass. Calculate by adding atomic masses from the Periodic Table.
What is relative molecular mass?
- Relative molecular mass (Mr) is the sum of the relative atomic masses of the atoms in a molecule.
- It shows how heavy one molecule is compared to one-twelfth of a carbon-12 atom.
- Chemists use Mr to compare molecule sizes.
What is relative formula mass?
- Relative formula mass (Mr) is used for ionic compounds.
- Ionic compounds do not form molecules but have formulas.
- Mr adds up the relative atomic masses of all atoms in the formula.
Using atomic mass from the periodic table
- Each element's relative atomic mass is on the Periodic Table.
- To find the Mr of a molecule, add the atomic masses of all atoms.
- For example, water (H2O) has Mr = 2(H) + 1(O) = 2 x 1 + 16 = 18.
Summary of calculations
- To calculate Mr, write the chemical formula.
- Find atomic masses from the Periodic Table.
- Add atomic masses, considering the number of atoms.
- Use Mr for molecules and relative formula mass for ionic compounds.
Test yourself on Relative Molecular Mass (Mr)
What is Relative Molecular Mass (Mr) defined as?
If a molecule has the formula XY2, how would you calculate its Mr?