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1.5.2Principles of Chemistry

Calculate Relative Mass

5 min read 5 parts Lesson 23 of 50 1 quiz

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Section 1 of 5Key Definitions

1.5.2 Calculate Relative Mass

Relative masses are numbers without units, which allow substances to be compared.

Relative atomic mass, Ar
  • The weighted average mass of an atom of an element compared with one twelfth of a carbon-12 atom, given on the periodic table.
Relative formula mass, Mr
  • The sum of the relative atomic masses of all the atoms shown in the formula of a compound or element.
Diatomic molecule
  • An element existing as two atoms joined together, such as H₂, so its Mr is twice its Ar.
Brackets
  • A subscript outside a bracket multiplies every atom inside it, so Ca(OH)₂ contains one calcium, two oxygen and two hydrogen atoms.

Section 2 of 5Step-by-Step Method

1
Write down the formula and count how many atoms of each element it contains, including those inside brackets.
2
Look up the relative atomic mass of each element on the periodic table, using the value given rather than a rounded number.
3
Multiply each relative atomic mass by the number of atoms of that element, applying any brackets correctly.
4
Add the results together to give the relative formula mass, which has no units.
SubstanceFormulaAtom countMr
WaterH₂OH: 2, O: 118
Carbon dioxideCO₂C: 1, O: 244
Calcium hydroxideCa(OH)₂Ca: 1, O: 2, H: 274
Aluminium sulfateAl₂(SO₄)₃Al: 2, S: 3, O: 12342

Section 3 of 5Using Relative Masses

Using Relative Masses
Conservation
  • In a balanced equation the total Mr of the reactants equals the total Mr of the products, which follows from conservation of mass.
Comparing masses
  • Mr values allow the masses of substances to be compared, and they are the basis of reacting mass calculations.
Hydrated salts
  • The Mr of a hydrated salt such as CuSO₄·5H₂O is the Mr of the anhydrous salt plus five times the Mr of water.
Ar values
  • Ar values are often not whole numbers, such as 35.5 for chlorine, so use the values given rather than rounding them.

Section 4 of 5Worked Example

Question

  • Calculate the relative formula mass of aluminium sulfate, Al₂(SO₄)₃.
  • Use Ar values Al = 27, S = 32 and O = 16.

  1. Count the atoms2 Al, 3 S, and 3 × 4 = 12 O.
  2. Add the contributions(2 × 27) + (3 × 32) + (12 × 16).
  3. Calculate54 + 96 + 192 = 342.

Answer The relative formula mass of aluminium sulfate is 342.

Section 5 of 5Exam Checklist & Tips

1.5.2 – Key Points for the Exam

  • Relative atomic mass (Ar) The weighted average mass of an atom of an element compared with one twelfth of carbon-12.
  • Relative formula mass (Mr) The sum of the relative atomic masses of the atoms in a formula.
  • Diatomic molecule A molecule made of two atoms of the same element, such as oxygen, O₂.
  • Law of Conservation of Mass The rule that mass is neither created nor destroyed in a chemical reaction.

Common Mistakes

  • Forgetting to multiply the atoms inside brackets by the subscript outside, for example in Ca(OH)₂.
  • Rounding Ar values to whole numbers when the periodic table gives values such as 35.5.
  • Adding units such as g to the relative formula mass, which is a ratio and has no units.
  • Using the mass number instead of the relative atomic mass from the periodic table.
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