Define relative molecular mass, $M_r$ as the sum of the relative atomic masses. Relative formula mass, $M_r$ will be used for ionic compounds

Stoichiometry: Relative Masses of Atoms and Molecules ⚛️

What is Relative Molecular Mass?

The relative molecular mass, written as \$M_r\$, is the total of the relative atomic masses of all atoms in a molecule. Think of it as the “weight” of a molecule when you add up the weights of each of its building blocks.

Mathematically:

\$Mr = \sum mi\$

Where \$m_i\$ is the relative atomic mass of the ith atom in the molecule.

Example: Water (H₂O) 🧪

Using the atomic masses (H = 1.008, O = 15.999):

\$Mr(\text{H}2\text{O}) = 2(1.008) + 15.999 = 18.015\$

So, one molecule of water has a relative mass of 18.015 atomic mass units (amu).

Relative Formula Mass for Ionic Compounds 🔬

For ionic compounds, we use the term relative formula mass instead of molecular mass because ions don’t form discrete molecules. The calculation is the same: sum the relative atomic masses of all atoms in the formula unit.

ElementNameRelative Atomic Mass (amu)
HHydrogen1.008
OOxygen15.999
NaSodium22.990
ClChlorine35.453

Exam Tip 📚

  • Always check the formula unit when calculating relative formula mass for ionic compounds.

  • Use the most recent periodic table values; atomic masses can be rounded to three decimal places.

  • Remember: \$M_r\$ is dimensionless (amu) – it’s a “relative” weight, not an absolute mass.

  • Practice with different compounds: e.g., CaCl₂, MgO, Al₂O₃ to build confidence.