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6.2.1Quantitative Chemistry

Relative Formula Mass

5 min read 5 parts Lesson 4 of 12 1 quiz

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Section 1 of 5What Mr Means

6.2.1 Relative Formula Mass

Relative formula mass compares the mass of a formula unit with one twelfth of the mass of a carbon-12 atom.

Symbol
  • Mr is the relative formula mass, and for simple molecules it is called the relative molecular mass.
Ionic compounds
  • For ionic compounds, Mr is found from the formula unit, such as NaCl, and is called the formula mass.
No units
  • Mr has no units because it is a ratio of masses, even though it is used with grams in later calculations.
Data source
  • Relative atomic masses, Ar, are taken from the Periodic Table and are added together for the atoms in the formula.

Section 2 of 5Calculating Mr Step by Step

Working methodically through each element avoids the most common errors.

1
Write the formula and count how many atoms of each element it contains.
2
Multiply the number of atoms of each element by its relative atomic mass, Ar.
3
Expand any brackets first, so that every atom inside the bracket is counted, such as the two NO₃ groups in Mg(NO₃)₂.
4
Add all the values together to give the Mr of the substance.

Section 3 of 5Common Mr Values

Common Mr Values
Water
  • H₂O(2 × 1) + (1 × 16) = 18.
Carbon dioxide
  • CO₂(1 × 12) + (2 × 16) = 44.
Calcium hydroxide
  • Ca(OH)₂40 + (2 × 16) + (2 × 1) = 74.
Brackets
  • A subscript outside a bracket multiplies every atom inside it, which is often missed in calculations.

Section 4 of 5Worked Example

Question Calculate the relative formula mass of aluminium sulfate, Al₂(SO₄)₃ (Ar: Al = 27, S = 32, O = 16).

  1. Count the atoms2 Al, 3 S and 12 O (3 sulfate groups, each with 4 O atoms).
  2. Multiply each by its Ar(2 × 27) + (3 × 32) + (12 × 16).
  3. Add the values54 + 96 + 192 = 342.

Answer Mr of Al₂(SO₄)₃ = 342

Section 5 of 5Exam Checklist & Tips

6.2.1 – Key Points for the Exam

  • Relative formula mass The sum of the relative atomic masses of all the atoms in a formula, shown as Mr.
  • Relative atomic mass The average mass of an atom of an element compared with one twelfth of the mass of a carbon-12 atom, shown as Ar.
  • Formula unit The simplest ratio of ions shown in the formula of an ionic compound, such as NaCl.
  • Relative molecular mass The Mr of a simple molecular substance, such as H₂O.

Common Mistakes

  • Forgetting to multiply by the number of atoms when a formula contains a subscript, such as H₂O.
  • Ignoring the subscript outside a bracket, such as counting only two O atoms in Ca(OH)₂ instead of two O and two H.
  • Using the mass number instead of the relative atomic mass from the Periodic Table.
  • Giving Mr a unit such as g, when the value is a ratio with no units.
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