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 P: 375 From van der Waals , (P+a/v^2)(v-b)=RT, At critical temperature, I get (∂P/∂V)at constant temperature =0 and (∂^2P/∂V^2) at constant temperature ,T=0. then critical pressure,P = a/(27b^2)--------1 critical volume,v=3b-----------2 critical temperature=8a/(27Rb)----------3 then simultaneous equation 1 and 3, I get b=(RT/8P), b=(v/3) ------------------4 But from the experiment, we get T,P,v and then substitute into the two equation from 4,both b have different values. Why? Thank you