Question

why pressure and temperature are mentioned also when the volume of a gas is mentioned

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Answer 1

The volume of a given amount of gas is inversely proportional to its pressure when temperature is held constant (Boyle's law). Under the same conditions of temperature and pressure, equal volumes of all gases contain the same number of molecules (Avogadro's law).

Answer 2

Because gases do not have a fixed volume. The volume of gases change at different temperatures and pressures.