second derivative of Pressure with respect to moles?

mnnob07

binary mixture.
Na=moles of a
Nb=moles of b

(using Peng Robinson Equation of state)

(second order partial derivative below)

d^2P/(dNa^2) holding T, molar volume, Nb constant

I can't figure out how to do this?

I know that Peng Robinson is a function of concentration of Na and Nb... but I tried to substitute all molar fractions with Na/(Na+Nb) & Nb/(Na+Nb) put the complete expression in Mathcad (was huge!) and it gives 0 as the first derivative of Na or Nb so something went wrong or it decided not to do it...

Attached is/was my Mathcad input and also the whole equation I am trying to get an expression of (this partial derivative is just part of it; don't mind the cursor on the right)

Attached Thumbnails