Degrees of freedom

A saturated solution of Na2SO4 with excess of the solid is present in equilibrium with its vapor in a closed vessel.  Calculate the number of degrees of freedom, f, of the system.  Ignor the autoprotolysis of water and assume that the salt solvates without ionizing.

F = c - p +2   or use  f = c-p +2 -r -a

c = chem species
p = phases
r = independant chemical reactions
a = additional restrictions