The mole: why it exists, and how to never mix up moles and molar mass again

Class 11 chemistry has one idea that every later chapter depends on, and it is taught as a number to memorise: 6.022 times ten to the twenty-three. Here is why that number exists, and the one habit that stops the mistakes.

The problem the mole solves: atoms are too small to count and too light to weigh one at a time. But chemistry is about counting, because reactions happen atom by atom. Two hydrogens for every oxygen, not two grams for every gram. So chemists needed a way to count by weighing.

The trick: one mole of anything is 6.022 times ten to the twenty-three particles of it, and that number was chosen so that one mole of a substance weighs, in grams, the same as its relative atomic or molecular mass. One mole of carbon-12 weighs 12 grams. One mole of water, molecular mass 18, weighs 18 grams. Now you can count atoms with a balance.

The three quantities students confuse:

Molar mass is a property of the substance. Water is always 18 grams per mole. It never changes with how much you have.

Moles is how much you have. Half a mole, three moles, 0.02 moles. It is a count.

Mass is what the balance shows. Mass equals moles times molar mass. That single equation, rearranged, is most of the numerical questions in the chapter.

The habit: write units on everything. If you write 36 g divided by 18 g per mol, the grams cancel and you are left with mol, and you know you have moles. If the units do not cancel to what the question asked for, you have set it up wrong, and you find out before the marker does.

Two traps. Molar mass of a molecule is the sum of its atoms: H2SO4 is 2 times 1 plus 32 plus 4 times 16, which is 98, not 49. And 'number of molecules' means moles times Avogadro's number, while 'number of atoms' means that again times the atoms per molecule. Read which one the question asked.

The worked problems are in the Class 11 Chemistry handwritten notes on the mole and stoichiometry.