Quantitative Aspects: The Mole

Atoms are far too small and numerous to count one by one, so chemists count them in a big fixed bundle called the mole (mol).

• One mole is $6{,}022 \times 10^{23}$ particles (Avogadro's number) — just as 'a dozen' always means 12.
• The molar mass ($M$) of a substance is the mass of one mole, in grams per mole ($\text{g·mol}^{-1}$). You read it off the periodic table: carbon is $12\ \text{g·mol}^{-1}$; for a compound, add up the atoms — water $\text{H}_2\text{O}$ is $2(1) + 16 = 18\ \text{g·mol}^{-1}$.

The key formula links number of moles ($n$), mass ($m$) and molar mass ($M$):

Worked example: how many moles are in $36\ \text{g}$ of water? $n = \frac{36}{18} = 2\ \text{mol}$.