Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

1.4.2Additional lessons

Molar Mass — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 15 of 172

📚 IB Chemistry HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين المول والحسابات الكيميائية - 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)
Word Equation & Conditions: عدد المولات = الكتلة بالجرام ÷ الكتلة المولية | عدد الجسيمات = عدد المولات × عدد أفوجادرو
Location & Specifications: كيمياء التحصيلي وثانوي وبكالوريا و Cambridge IGCSE Chemistry.
0%read

Section 1 of 5Molar Mass

The molar mass is the mass of one mole of a substance.

Definition
  • Definition Molar mass M has units g mol⁻¹ and is numerically equal to the relative atomic or formula mass.
Elements
  • Elements For an element, M is the relative atomic mass in g mol⁻¹, such as 12.01 for carbon.
Compounds
  • Compounds For a compound, add the relative atomic masses of all the atoms in the formula.
Hydrates
  • Hydrates Include water of crystallisation, so CuSO₄·5H₂O has the mass of five water molecules added.

Section 2 of 5Mass and Moles

The equation n = m ÷ M converts between mass and amount.

1
Write down the formula and use the periodic table to find the relative atomic masses of the elements.
2
Add up the masses of all atoms in the formula to find M.
3
To find moles, divide the mass in grams by the molar mass.
4
To find mass, multiply moles by molar mass.
5
Use significant figures that match the data, and include the unit in the final answer.

Section 3 of 5Worked Values

Worked Values
Water
  • Water M(H₂O) = 2 × 1.01 + 16.00 = 18.02 g mol⁻¹.
Calcium carbonate
  • Calcium carbonate M(CaCO₃) = 40.08 + 12.01 + 3 × 16.00 = 100.09 g mol⁻¹.
Bracketed formula
  • Bracketed formula In Ca(OH)₂ the bracket multiplies both atoms, so there are two O and two H.
Check
  • Check The answer should be sensible: a few grams of a compound typically corresponds to a few hundredths of a mole.
SubstanceM / g mol⁻¹
H₂O18.02
CO₂44.01
NaCl58.44

Section 4 of 5Worked Example

Question Calculate the amount of CaCO₃ in 5.00 g, and the number of formula units.

  1. M(CaCO₃) = 40.08 + 12.01 + 3 × 16.00 = 100.09 g mol⁻¹.
  2. n = 5.00 ÷ 100.09 = 0.0500 mol.
  3. Number = 0.0500 × 6.02 × 10²³ = 3.01 × 10²² formula units.

Answer 0.0500 mol; 3.01 × 10²² formula units

Section 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 in a formula unit.
  • Water of crystallisation Water molecules that are part of the crystal structure of a hydrate.
  • Formula unit The simplest ratio of ions in an ionic compound.

Common Mistakes

  • Forgetting to multiply atoms inside a bracket by the subscript outside it.
  • Using kg or mg instead of g when applying n = m ÷ M.
  • Confusing molar mass, with units, and relative molecular mass, which has none.
  • Dropping the water of crystallisation from the molar mass of a hydrate.

Last updated:

\n