What does Avogadro's law state about gases?

Avogadro's law is a fundamental principle in chemistry that states that the volume of a gas is directly proportional to the number of moles of gas present when the temperature and pressure are held constant. This relationship implies that if you increase the number of gas molecules (or moles) in a container while keeping the temperature and pressure steady, the volume occupied by the gas will also increase. Conversely, reducing the number of moles will lead to a decrease in volume under the same conditions. This principle is essential in understanding gas behavior and is fundamental to the concept of molar volume at standard temperature and pressure (STP), which is 22.4 liters for one mole of a gas.

The other choices do not accurately represent Avogadro's law. The law primarily focuses on the relationship between volume and the amount of gas (moles), rather than changes in pressure or temperature, which are addressed by other gas laws like Boyle's law and Charles's law. Thus, the emphasis on moles in option C directly aligns with the core of Avogadro’s law.