What a mole is
The definition begins from the same substance the chapter began with: carbon-12.
The mole
The amount of a substance containing as many elementary units (atoms, molecules, ions) as there are atoms in exactly 12.00 g of the 12C isotope is defined as one mole of that substance.
How it connects to the constant
The number of elementary units contained in a mole of a substance is a constant, and it is equal to the Avogadro constant.
1 mol = 6.022 × 1023
So a mole can be obtained by weighing
The number of units in a mole is so large that counting them is out of the question, so another method is used: to take one mole of atoms of an element, weigh out its relative atomic mass in grams.
| Substance | Ar or Mr | One mole is |
|---|---|---|
| Sodium atoms | 23 | 23 g of sodium |
| Glucose molecules | 180 | 180 g of glucose |
Say a mole of what
Saying "a mole" is not enough — you must say a mole of which particle. One mole of oxygen atoms is 16 g, while one mole of oxygen molecules (O2) is 32 g. Same word, two different masses.