Overview
This lecture explains how to calculate relative formula mass (Mr) and percentage mass of elements in compounds, with worked examples.
Relative Formula Mass (Mr)
- Relative formula mass (Mr) is the total of the relative atomic masses (Ar) of all atoms in a compound’s formula.
- Relative atomic mass (Ar) is the weighted average mass of all isotopes of an element.
- To calculate Mr, add up the Ar values of each atom in the compound, each multiplied by the number of atoms present.
- Example: MgCl₂ — Mr = 24 (Mg) + 2 × 35.5 (Cl) = 95.
- Example: H₂SO₄ — Mr = 2 × 1 (H) + 1 × 32 (S) + 4 × 16 (O) = 98.
Calculating Percentage Mass of an Element in a Compound
- Percentage mass = (Ar of element × number of atoms) ÷ Mr of compound × 100.
- Example: Sulfur in H₂SO₄: (32 × 1) ÷ 98 × 100 = 32.7%.
- Example: Oxygen in Ca(OH)₂: (16 × 2) ÷ 74 × 100 = 43.2%.
Key Terms & Definitions
- Relative atomic mass (Ar) — The average mass of an element's isotopes, weighted by natural abundance.
- Relative formula mass (Mr) — The sum of the relative atomic masses of the atoms in a compound's formula.
- Isotope — Atoms of the same element with different numbers of neutrons.
Action Items / Next Steps
- Practice calculating Mr and percentage mass for other compounds as homework.
- Review periodic table values for Ar for common elements.