Is Total Molar Concentration Constant in an Isothermal System?

[mech-eng]

how can we understand total molar concentration remains constant in a isobaric isothermal system? if it is from perfect gas equation pv=nrt although p and t are constant but v isn't constant so since mol number can be change over volume, total molar concentration may change.it is a situation of a tube with one end is surrounded by gaseous nitrogen and one end is closed with a solid naphthalene in equilibrium with its vapor.

 

[Hyo X]

Total molar concentration remains constant because n can change
probably: if it is solid napthalene in equilibrium with its vapor, then napthalene atoms can desorb and enter the gas phase to maintain equilibrium. so if volume changes, n will change correspondingly

 

[mech-eng]

But it must always be constant which is equal to 1 so we shouldn't limit the situation to the isobaric isothermal case.Am I right? is C=C_A+C_B always true or only for at constant
T and P