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1.4.2Additional lessons

Molar Mass — IB Chemistry SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 12 of 98

📚 IB Chemistry SL Hub / Topical Revision
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Curriculum Equation Key Moles & Stoichiometry FormulasFast Exam Solution
n = m / M_r | N = n × N_A (6.022 × 10²³) | M = n / V | V_gas = n × 22.4 L (STP)
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Part 1 of 5Relative and Molar Masses

1.4.2 Molar Mass

Mass on the atomic scale connects to laboratory masses by the mole.

Relative atomic mass
  • The weighted mean mass of an atom compared with one twelfth of the mass of a carbon-12 atom, with no units.
Relative formula mass
  • The sum of the relative atomic masses of all atoms in the formula, also without units.
Molar mass M
  • The mass of one mole of a substance, with the same number as the relative mass but in g mol⁻¹.
Data source
  • Values of relative atomic mass come from the periodic table in the data booklet.

Part 2 of 5Calculation Steps

Convert between mass and moles with n = m ÷ M.

1
Write the formula and list the atoms of each element, including those inside brackets multiplied by a subscript.
2
Add up the relative atomic masses to obtain M in g mol⁻¹.
3
To find moles, divide the mass in grams by M; to find mass, multiply moles by M.
4
Keep the number of significant figures in the answer consistent with the data given.

Part 3 of 5Hydrates and Brackets

Hydrates and Brackets
Brackets
  • In Ca(OH)₂ the subscript 2 multiplies everything inside the bracket, giving one Ca, two O and two H.
Hydrated salts
  • In CuSO₄·5H₂O the water of crystallisation is included, so M includes five H₂O units.
Mass in grams
  • The equation needs the mass in grams, so kilogram or milligram values must be converted first.
Units
  • The mole is the unit of amount and g mol⁻¹ is the unit of molar mass.

Part 4 of 5Worked Example

Question Calculate the amount in moles in 37.05 g of calcium hydroxide, Ca(OH)₂.

  1. M = 40.08 + 2 × (16.00 + 1.01) = 74.10 g mol⁻¹.
  2. n = m ÷ M = 37.05 ÷ 74.10.
    What the symbols mean: n = amount of substance (mol) · m = mass (g) · M or Mr = molar mass (g mol−1, numerically equal to Mr)

Answer 0.500 mol

Part 5 of 5Exam Checklist & Tips

1.4.2 – Key Points for the Exam

  • Molar mass The mass of one mole of a substance, in g mol⁻¹.
  • Relative formula mass The sum of the relative atomic masses of the atoms in a formula.
  • Amount The quantity in moles, found by n = m ÷ M.
  • Water of crystallisation Water molecules chemically bound into a crystal structure in a fixed ratio.

Common Mistakes

  • Forgetting to multiply the atoms inside brackets by the subscript.
  • Using the mass in kg or mg without converting to g.
  • Giving molar mass without units, or with the unit g.
  • Mixing up the rearrangement as n = M ÷ m.

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