01Why chemists count in moles
Atoms are far too small to count one by one, so chemists count them in groups. One mole is 6.022 × 1023 particles (Avogadro's number), the same way a dozen is 12.
The key link: the molar mass of an element in grams equals its average atomic mass in amu. One carbon atom is about 12.01 amu, so one mole of carbon weighs 12.01 g.
02Notebook box
n = m / M (moles = grams ÷ molar mass). N = n × 6.022 × 1023 (particles).
Worked example: How many molecules are in 9.0 g of H₂O? → 9.0 ÷ 18.02 = 0.50 mol → 0.50 × 6.022 × 1023 = 3.0 × 1023 molecules.