From van der Waals , (P+a/v^2)(v-b)=RT,(adsbygoogle = window.adsbygoogle || []).push({});

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

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# Van der Waals gas is not real gas?

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