15.1 The mole
- Syllabus
- 9702–2028–2029
- Topic
- 15.1
- Level
- A2
Learning objectives
15.1.1Amount of substance is an SI base quantity with the base unit mol• understand that amount of substance is an SI base quantity with the base unit mol15.1.2Molar quantities where one mole of any substance is the amount containing a• use molar quantities where one mole of any substance is the amount containing a number of particles of that substance equal to the Avogadro constant NA
Amount of substance is an SI base quantity measured in moles
| Role | Correct term |
|---|---|
| SI base quantity | amount of substance |
| quantity symbol | n |
| SI base unit | mole |
| unit symbol | mol |
Amount of substance provides a macroscopic measure of how many specified microscopic entities are present. Always name the entity: atom, molecule, ion, electron or formula unit.
One mole of H₂O means one mole of H₂O molecules. It contains two moles of H atoms and one mole of O atoms only after the molecular composition is used.
The mole is a unit, not a particle or a mass unit. Different substances can have the same amount in mol but different masses because their molar masses differ.
Use Avogadro's constant to connect mass, moles and particles
One mole of any substance is the amount containing a number of specified entities equal to the Avogadro constant N_A. Thus N_A is the number of entities per mole.
NA=6.02×1023mol−1
n=m/Mm,N=nNA,n=N/NA,massofoneentity=Mm/NA
Use compatible mass units: if m is in kg, use molar mass M_m in kg mol⁻¹; if m is in g, use g mol⁻¹. N is a dimensionless count, while n is in mol.
0.50 mol of electrons contains N=(0.50)(6.02×10²³)=3.01×10²³ electrons. A gas sample containing 3.4×10²² molecules has n=(3.4×10²²)/(6.02×10²³)=5.65×10⁻² mol.
N_A is not a mass and does not depend on substance. Specify whether N counts atoms, molecules, ions, electrons or formula units; one mole of NaCl means N_A formula units, not N_A grams.