The volume of a gas at a given temperature and pressure is directly proportional to the quantity of the gas. Avogadro’s hypothesis comes directly from this relationship, and you should definitely remember the following statement (which is an extrapolation of the above idea) for the SAT II Chemistry test: Equal volumes of gases under the same conditions of temperature and pressure contain equal numbers of molecules. Avogadro’s law, which is derived from this basic idea, says that the volume of a gas maintained at constant temperature and pressure is directly proportional to the number of moles of the gas, or