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1.2.1Physical Chemistry

Relative Atomic Mass and Relative Molecular Mass — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 8 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6The Carbon-12 Standard

Atoms are far too light to weigh individually, so masses are compared with a standard.

Standard
  • Standard The mass of a carbon-12 atom is the reference, and one-twelfth of it is the unified atomic mass unit.
Relative atomic mass
  • Relative atomic mass Ar is the weighted mean mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom.
Why weighted
  • Why weighted Elements are mixtures of isotopes, so the mass is averaged using the abundance of each isotope.
No units
  • No units Relative masses are ratios, so the units cancel and Ar has no unit.

Part 2 of 6Isotopic and Molecular Masses

Isotopic and Molecular Masses
Relative isotopic mass
  • Relative isotopic mass The mass of an atom of one isotope compared with one-twelfth of the mass of a carbon-12 atom.
Relative molecular mass
  • Relative molecular mass Mr is the weighted mean mass of a molecule compared with one-twelfth of the mass of a carbon-12 atom, found by adding Ar values.
Relative formula mass
  • Relative formula mass Mr of ionic or giant covalent compounds is found for the simplest formula, for example NaCl or SiO₂.
Same method
  • Same method Relative molecular mass and relative formula mass are calculated in the same way, by adding the Ar values of all atoms in the formula.

Part 3 of 6Calculating Mr

1
Write down the formula of the substance, including the number of each type of atom.
2
Look up the Ar of each element in the periodic table.
3
Multiply each Ar by the number of atoms of that element in the formula.
4
For brackets, multiply everything inside the bracket by the number outside it.
5
Add the totals together, and give the answer without units.

Part 4 of 6Using Ar Values

Chlorine
  • Chlorine Chlorine is 75% ³⁵Cl and 25% ³⁷Cl, so Ar = (35 × 75 + 37 × 25) ÷ 100 = 35.5.
Rounding
  • Rounding Use the Ar values given in the exam question, usually to one decimal place, for example H = 1.0 and O = 16.0.
Hydrates
  • Hydrates For hydrated salts add the Mr of each water of crystallisation, as in CuSO₄·5H₂O.
Check
  • Check A formula mass must be greater than the Ar of any individual element in the compound.

Part 5 of 6Worked Example

Question Calculate the relative formula mass of ammonium sulfate, (NH₄)₂SO₄ (Ar: N = 14.0, H = 1.0, S = 32.1, O = 16.0).

  1. Atoms in (NH₄)₂SO₄N = 2, H = 8, S = 1, O = 4.
  2. Multiply
    • 2 × 14.0 = 28.0;
      • 8 × 1.0 = 8.0;
      • 1 × 32.1 = 32.1;
      • 4 × 16.0 = 64.0.
  3. Add28.0 + 8.0 + 32.1 + 64.0 = 132.1.

Answer Mr of (NH₄)₂SO₄ = 132.1

Part 6 of 6Exam Checklist & Tips

1.2.1 – Key Points for the Exam

  • Unified atomic mass unit One-twelfth of the mass of an atom of carbon-12.
  • Relative atomic mass The weighted mean mass of an atom of an element relative to one-twelfth of the mass of carbon-12.
  • Relative formula mass The weighted mean mass of the formula unit of a compound relative to one-twelfth of the mass of carbon-12.
  • Isotope An atom of an element with a particular mass number.

Common Mistakes

  • Forgetting to multiply by the number outside a bracket, for example only counting one N in (NH₄)₂SO₄.
  • Giving units such as g or g mol⁻¹ for relative masses.
  • Defining Ar with the mass of a hydrogen atom instead of one-twelfth of carbon-12.
  • Leaving out “weighted mean” in the definition of relative atomic mass.

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