Amount of Substance Essentials

The mole is chemistry's way of counting particles - think of it like a dozen, but instead of 12, it's 6.02 × 10²³ particles (that's Avogadro's constant). This massive number lets us work with atoms and molecules in manageable quantities.

Two key formulas will sort most of your calculations: moles = mass ÷ Mr (where Mr is relative molecular mass) and moles = concentration × volume. These connect the microscopic world of atoms to the macroscopic measurements you can actually make in the lab.

Percentage yield tells you how efficient your reaction was - it's actual yield divided by theoretical yield, times 100. Atom economy is different - it measures how many atoms from your reactants actually end up in your desired product, helping you spot wasteful reactions.

The ideal gas equation PV = nRT connects pressure, volume, moles, and temperature for gases. Remember your units: pressure in pascals, volume in m³ or dm³, and temperature in Kelvin.

Quick tip: Always check your units match the formula - most mistakes come from mixing up dm³ and cm³!

Relative atomic mass (Ar) and relative molecular mass (Mr) compare atoms and molecules to 1/12 of a carbon-12 atom. These give you the numbers you need for the periodic table and your mole calculations.

Empirical formulas show the simplest ratio of atoms, whilst molecular formulas show the actual numbers. To find molecular formula from empirical: calculate the empirical formula's Mr, divide the actual Mr by this, then multiply all subscripts by your answer.