24. Relative Formula Mass & Percentage Mass Calculations

1. Relative Atomic Mass (Ar)

  • Definition: The average mass of all isotopes of an element, taking into account how common each one is.

  • Notation: Written as Ar.

  • Example: Chlorine has two main isotopes (35 and 37). Its relative atomic mass is 35.5.

2. Relative Formula Mass (Mr)

  • Definition: The total average mass of an entire compound.

  • Notation: Written as Mr.

  • How to calculate: Add together the relative atomic masses (Ar) of all the atoms in the chemical formula.

  • Examples:

    • Magnesium Chloride (MgCl2): * 1 x Magnesium (24) + 2 x Chlorine (35.5)

      • 24 + (35.5 * 2) = 95

    • Sulfuric Acid (H2SO4): * 2 x Hydrogen (1) + 1 x Sulfur (32) + 4 x Oxygen (16)

      • (2 1) + 32 + (4 16) = 98

3. Percentage Mass of an Element

To find what percentage of a compound's total mass comes from a specific element, use this formula:

% Mass = [(Ar of element × number of atoms of that element) / Mr of the compound] × 100

  • Example: Percentage mass of Sulfur in Sulfuric Acid (H2SO4):

    • Ar of Sulfur = 32 | Atoms of Sulfur = 1 | Mr of H2SO4 = 98

    • (32 * 1) / 98 = 0.327

    • 0.327 * 100 = 32.7%

  • Example: Percentage mass of Oxygen in Calcium Hydroxide (Ca(OH)2):

    • Ar values: Ca = 40, O = 16, H = 1.

    • Total atoms: 1 Calcium, 2 Oxygen, 2 Hydrogen (the 2 outside the bracket applies to everything inside).

    • Top of formula: 16 * 2 = 32.

    • Mr of compound: 40 + (16 2) + (1 2) = 74.

    • (32 / 74) * 100 = 43.2%