1.4.1Additional lessons
The Mole Unit — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 14 of 172
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.
Section 1 of 5Counting by the Mole
Atoms and molecules are far too small to count one by one, so chemists count them in very large groups.
Mole
- Mole One mole is the amount of substance containing the same number of elementary entities as there are atoms in 12 g of carbon-12.
Avogadro constant
- Avogadro constant Equal to 6.02 × 10²³ mol⁻¹, it is the number of particles in one mole.
Particles
- Particles The entity can be an atom, molecule, ion, electron or formula unit, so it must be stated.
Symbol
- Symbol Amount of substance is represented by n and measured in mol.
Section 2 of 5Conversions
The relationship N = n × NA links number of particles and amount.
1
To find the number of particles, multiply moles by 6.02 × 10²³.
2
To find moles from particles, divide the number of particles by 6.02 × 10²³.
3
For compounds, count the atoms of each element in the formula to find the number of atoms of each element.
4
For example 1 mol of CO₂ contains 1 mol of carbon atoms and 2 mol of oxygen atoms.
5
Always state which particle is being counted, such as molecules or atoms, to avoid ambiguity.
Section 3 of 5Scale and Meaning
Scale and Meaning
Large number
- Large number The Avogadro constant is so large that a mole of grains of sand would cover a huge area to a great depth.
Link to mass
- Link to mass Molar mass in g mol⁻¹ links the number of moles to the mass that can be weighed.
Standard
- Standard Carbon-12 is the reference for the mole and for relative atomic mass.
Use
- Use Moles allow reacting quantities to be compared in ratios from equations.
Section 4 of 5Worked Example
Question How many oxygen atoms are in 0.250 mol of CO₂?
- Molecules of CO₂ = 0.250 × 6.02 × 10²³ = 1.505 × 10²³.
- Each molecule has 2 oxygen atoms, so atoms = 2 × 1.505 × 10²³ = 3.01 × 10²³.
Answer 3.01 × 10²³ oxygen atoms
Section 5 of 5Exam Checklist & Tips
1.4.1 – Key Points for the Exam
- Mole The amount of substance containing 6.02 × 10²³ elementary entities.
- Avogadro constant 6.02 × 10²³ mol⁻¹.
- Amount of substance The quantity measured in moles, symbol n.
- Elementary entity The particle being counted: atom, molecule, ion or electron.
Common Mistakes
- Counting molecules when the question asks for atoms.
- Treating the mole as a mass instead of a number of particles.
- Leaving off the unit mol⁻¹ from the Avogadro constant.
- Dividing instead of multiplying when converting moles to particles.
Last updated:
\n