Chemistry · Basic concepts. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Chemists count particles by weighing. The link between mass and number is the mole: the atomic mass, molecular mass or formula mass of a substance taken in grams. So 1 mole of carbon is 12 g, 1 mole of water is 18 g and 1 mole of NaCl is 58.5 g.
One mole of any substance contains the same number of particles, 6.02 × 1023. This is Avogadro's number (NA).
For gases there is a third link. One mole of any ideal gas occupies 22.414 dm3 at STP (0 °C and 1 atm). This is the molar volume. Every calculation goes through moles first: change what you are given into moles, then into what you want.
Students stop at the number of molecules when the question asks for atoms or ions. Read the last word of the question, then multiply by the number of atoms or ions in one formula.
Answer: 6.02 × 1022 hydrogen atoms.
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