Final answer:
Avogadro's law states that the volume of a gas is proportional to the moles of the gas when pressure and temperature are kept constant.
Step-by-step explanation:
The law that states that the volume of a gas is proportional to the moles of the gas when pressure and temperature are kept constant is called Avogadro's law. According to this law, if the pressure and temperature of a gas are constant, the volume of the gas will vary directly with the number of gas molecules present.