At constant temperature and pressure, the volume of a gas is directly proportional to the number of molecules. This is called _______.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Avogadro's gas law
Avogadro’s law (1811) states that at constant temperature and pressure, equal volumes of all gases contain equal numbers of molecules; in other words, the volume of a gas is directly proportional to the number of molecules (or moles): \(V\propto n\).
Why the other options are incorrect
Option (1) Ideal gas equation — Incorrect. \(PV=nRT\) combines all the gas laws in one equation; it is not the single law described.
Option (3) Charles's law — Incorrect. At constant pressure, the volume of a fixed amount of gas is proportional to its absolute temperature.
Option (4) Boyle's law — Incorrect. At constant temperature, the volume of a fixed amount of gas is inversely proportional to its pressure.