The answer is: Volume is directly proportional to the number of moles.
Given the ideal gas equation: PV = nRT
We can see that pressure (P) and volume (V) are on the left side of the equation, while the number of moles (n), the gas constant (R), and temperature (T) are on the right side.
If you increase/decrease the volume, this results in a direct increase/decrease of the number of moles, which proves the statement true. Volume is also directly proportional to temperature. Pressure is inversely proportional to volume, and directly proportional to the number of moles.
