(40) GCSE Chemistry Revision "Relative Formula Mass"

Understanding Relative Formula Mass

Definition

• Relative Formula Mass: The sum of the relative atomic masses of the atoms in a chemical formula.

• Represented by the symbol: Mr.

Key Points

• No Units: Relative formula mass is dimensionless and has no associated units.

• No Big Numbers: Calculations do not involve large numbers, regardless of the number of molecules.

Relative Atomic Mass

Definition

• The average mass of the isotopes of an element, weighed according to their abundance.

Example: Chlorine

• Isotopes: Chlorine has two main isotopes:

  • Chlorine-35: Mass number is 35 (17 protons, 18 neutrons).

  • Chlorine-37: Mass number is 37 (17 protons, 20 neutrons).

• Periodic Table Representation: Average mass is shown as 35.5 due to the relative abundance of each isotope, with chlorine-35 being more prevalent.

Calculation Examples

Example 1: Methane (CH₄)

• Formula: CH₄

• Atoms in Methane:

  • 1 Carbon (C) : Relative Atomic Mass = 12

  • 4 Hydrogen (H) : Relative Atomic Mass = 1

• Calculation:

  • Total = (1 x 12) + (4 x 1) = 12 + 4 = 16

• Relative Formula Mass: 16 (no units)

Example 2: Calcium Sulfate (CaSO₄)

• Formula: CaSO₄

• Atoms in Calcium Sulfate:

  • 1 Calcium (Ca) : Relative Atomic Mass = 40

  • 1 Sulfur (S) : Relative Atomic Mass = 32

  • 4 Oxygen (O) : Relative Atomic Mass = 16

• Calculation:

  • Total = (1 x 40) + (1 x 32) + (4 x 16) = 40 + 32 + 64 = 136

• Relative Formula Mass: 136 (no units)

Example 3: Magnesium Hydroxide (Mg(OH)₂)

• Formula: Mg(OH)₂

• Atoms in Magnesium Hydroxide:

  • 1 Magnesium (Mg) : Relative Atomic Mass = 24

  • 2 Oxygen (O) : Relative Atomic Mass = 16

  • 2 Hydrogen (H) : Relative Atomic Mass = 1

• Calculation:

  • Total = (1 x 24) + (2 x 16) + (2 x 1) = 24 + 32 + 2 = 58

• Relative Formula Mass: 58 (no units)

Final Notes

• You should now understand relative formula mass and how to calculate it for different compounds.