Amedeo Avogadro tried to combine conclusions of Dalton’s atomic theory and Gay Lussac’s law of combining volumes is known as Avogadro law.

Avogadro law states that equal volumes of all gases under the same conditions of temperature and pressure contain an equal number of molecules.

Avogadro law means that as long as the temperature and pressure remain constant, the volume depends upon the number of molecules of the gas or in other words the amount of the gas.  

The number of molecules in one mole of a gas has been determined to be 6.022 ×1023 and is known as Avogadro constant.

One mole of each gas at standard temperature and pressure (STP) will have the same volume.

Standard temperature and pressure means 273.15 K (0°C) temperature and 1 bar (i.e., exactly 105  pascal) pressure.

At STP molar volume of an ideal gas or a combination of ideal gases is 22.71098 L mol–1.

Number of moles of a gas can be calculated by dividing mass by molecular mass.

The density of a gas is directly proportional to its molar mass.