Counting Chemical Particles with the Mole
Atoms, molecules, ions, and formula units are far too small to count one by one. The mole is a counting unit that connects a manageable amount of substance with a fixed, extremely large number of microscopic particles.
1. Amount of substance
Amount of substance is a physical quantity that measures how many specified elementary entities are present. Its symbol is (n), and its SI unit is the mole, symbol mol. The entity must always be named: atoms of Fe, molecules of H2O, ions of Cl−, or formula units of NaCl.
2. The Avogadro constant
The Avogadro constant is the number of specified entities in exactly one mole:
The exponent 23 means the decimal point is shifted 23 places. Scientific notation is essential because ordinary samples contain vast numbers of particles. The constant has unit mol−1, meaning “entities per mole.”
3. Two conversion equations
From moles to particles
\(N=nN_A\)
Multiply the amount (n) by (N_A). The result (N) is a count and therefore has no unit.
From particles to moles
\(n=\dfrac{N}{N_A}\)
Divide the particle count (N) by (N_A). The result is measured in mol.
4. Specify what is counted
| Substance | One mole contains | Important interpretation |
|---|---|---|
| He | (N_A) He atoms | A monatomic element is counted as atoms. |
| O2 | (N_A) O2 molecules | This also contains (2N_A) oxygen atoms. |
| H2O | (N_A) H2O molecules | This contains (2N_A) H atoms and (N_A) O atoms. |
| NaCl | (N_A) formula units | This corresponds to (N_A) Na+ ions and (N_A) Cl− ions. |