vapour-liquid system

[mojo4king]

Ive been trying to figure this one out for too long,please help :s !!!

Molar volume of a saturated vapour-liquid system is given by V=Vf(1-X) + VgX,
where Vf=molar vol. of condensed phase
Vg=molar volume of vapour phase
X=quality
A given closed system contains only water in VLE at 25oC. Vapour pressure of water at this temp. is 3168 Pa.

Calculate the molar volume of the system if the quality is 0.2.

Any ideas??
Thanks

[Astronuc]

This seems like a homework problem.

Firstly, one must understand the definition of quality, x.

x=mv/(mv + ml)

where mv = mass of vapor and ml = mass of liquid

So quality is defined on a mass basis, which can be related to moles.

Also the volume of either liquid or vapor is simply mass*specific volume and liquid and vapor have different specific volumes depending upon temperature and pressure. Specific volume is simply the reciprocal of density.