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Is this right?

(p+a(n/v)

^{2})(V-nb)=nRT

Just differentiate both sides with respect to P?

If I do that I get

0=V+P*(dV/dP)

_{t}-nb-(an

^{2})/V

^{2}*(dV/dP)

_{t}+2abn

^{3}/V

^{3}*(dV/dP)

_{t}

(dV/dP)

_{t}=(nb-V) / (P-n

^{2}a/V+2an

^{3}ab/V

^{3})

And now to calculate the bulk modulus:

k

_{t}=-1/V*(dV/dP)

_{t}= -1/V * (nb-V) / (P-n

^{2}a/V+2an

^{3}ab/V

^{3})

And one more questions:

What happens to the bulk modulus at the critical point? Is it equal to zero?