Reactions & the mole

How chemists count atoms by weighing them: the mole, the equation n = m/M, relative atomic mass, and working out empirical and molecular formulas.

Atoms are far too small and too many to count, so chemists count them by weighing. The mole is the bridge: a fixed, huge number of particles (Avogadro's number) that turns grams into a count.

This track uses the core equation n = m/M to move between mass and amount, explains why relative atomic mass is often not a whole number (isotope averaging, so chlorine is 35.5), and shows how to find empirical and molecular formulas from composition.

What you will learn:

  • The mole and Avogadro's number
  • The mole in practice: converting formula to grams and back
  • Relative atomic mass and why chlorine is 35.5
  • Empirical and molecular formulas

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Frequently asked questions

What is a mole in chemistry?
A mole is a fixed count of particles, about 6.022 x 10^23, chosen so that the mass in grams of one mole equals the substance's relative formula mass. It lets chemists count atoms by weighing.
Why is the relative atomic mass of chlorine 35.5?
Natural chlorine is a mixture of isotopes, mostly chlorine-35 and chlorine-37. The value 35.5 is the weighted average of those isotope masses, so no single atom actually weighs 35.5.