Volume is the independent variable in the PFR design equation, so you will want to vary volume until you achieve the desired conversion. The easiest way to do this is to choose a large volume (in this case, a final volume of 500 dm3 might be a good place to start), calculate your conversion as a function of volume, and then plot conversion versus volume. Remember, the point you're looking for in the example is the volume where conversion equals 80 percent of the equilibrium conversion, or a conversion of 71.1 percent (i.e., X =0.711).