Under what conditions does the molar volume of a gas decrease

The volume decreases in the same ratio as the ratio of pressure increases. But you won't get any change in volume.

Gases are compressible, meaning that when put under high pressure, the particles are forced closer to one another. This decreases the amount of empty space and reduces the volume of the gas. Gas volume is also affected by temperature. When a gas is heated, its molecules move faster and the gas expands.