Quantative Chemistry, could itself be a single catagory since it covers a broad range of topics linked by a common theme (in this case numbers). But at GCSE level, the number of techniques required is small and cann be easily contained within a single page.
The Mole
Avogadro's Gas Law
The law states that at the same temperature and pressure, the same volumes of different gasescontain the same number of particles.
So in 20dm3 of CO2 for example, there will be the same number of particles in 20dm3 of SO4, assuming they're in the same environment.