I am trying to write the equation Partsolmolsolv := Vm-x2(thedVmdx2) but it keeps telling me x2 must be a function but has the form: unitless. I would appreciate any help. Thanks!
i think you may lose the multiplication symbol between x2 and thedvmdx2，also you put thedvmdx2 in brackets，so mathcad think that means y(x) function
in addition，you should pay attention to the unit，Vm has unit mL/mol，but x2 and thedvmdx2 have no unit
See the attached,In addition to using units consistently, as per the previous comment, you also need to follow though with the indexes, or just vectorize the equations.