One way to gain a physical understanding of b in the van der Waals equation is to calculate the "excluded volume." Assume that the distance of closest approach between two similar spherical molecules is the sum of their radii (2r). (a) Calculate the volume around each molecule into which the center of another molecule cannot penetrate. (b) From your result in (a) calculate the excluded volume for one mole of molecules, which is the constant b. How does this compare with the sum of the volumes of 1 mole of the same molecule?