How do you obtain the Van der Waal's equation of state with respect to pressure?

I'm not sure if I understand the question, but I believe you're asking how to convert van der Waals' equation to be able to calculate true pressure (P-true) of a non-ideal gas.

Van der Waals' equation is:

P-ideal = [P-true + (n²a/V²)][V - nb] = 0.0821054nT

P-true + (n²a/V²) = 0.0821054nT / [V - nb]

P-true = {0.0821054nT / [V - nb]} - (n²a/V²)

Where:
P-ideal = pressure of an ideal gas
P-true = pressure of a gas under non-ideal conditions
V = volume in liters
n = moles
T = absolute temperature in kelvins
a,b = van der Waals' constants

I worked this one out a few weeks ago.  I also rewrote the equation to calculate the volume of a gas under non-ideal conditions for any values of n, T, and P.  The only problem is that the sucker is a cubic equation so you either have to solve for V using a graphing calculator, or plug in the numerical coefficients into an on-line equation solver like www.akiti.ca/Quad3Deg.html.

Hope I answered the question correctly.