Isothermal Expansion of an Ideal Gas

If we have one mole of an ideal gas in our cylinder, and keep the temperature constant at a temperature T₁ , then the gas will obey the ideal gas equation.
pV = RT₁ = constant .........(1)
Thus the equation for the pressure of an ideal gas during an isothermal expansion is
p = constant/V .......(2)
and we see that the pressure decreases as 1/V. This decrease is shown in the pV diagram of Figure (1).

Figure 1: In the isothermal expansion of an ideal gas, we have pV = constant. Thus the pressure decreases as 1/V.