The ideal gas law is PV = nRT, where P is the pressure, V is the volume, n is the number of moles, R is the molar gas constant and T is the absolute temperature. In order for any equation to be correct, its dimensions, or the units on both sides of the equation, must be correct. So if the moles are measured in mol, and the temperature in K, then the units of the molar gas constant tell us that the volume is being measured in liters and the pressure is being measured in atmospheres.