For one mole of a van der Waals' gas when b = 0 and T = 300 K, the pV vs 1V plot is shown below. The value of the van der Waals' constant a (atm L mol-2)

van der Waals' equation of state isp+n2aV2(V-nb)=nRTFor one mole and b = 0 the above equation condenses top+aV2V=RT⇒pV=RT-aV----------(i)Eq. (i) is a straight line equation between pV and 1V whose slope is -aEquating with slope of the straight line given in the graph-a=20.1-2163-2=-1.5⇒a = 1.5