Water / Steam evaporation calculation

[FloidRose]

Hello!

I'm trying to build simulation of steam generation process

My problem is this:

I want to know how to calculate pressure in the following problem-

closed container, V=1 litres
121 C water, 1 g
121 C saturated steam 1.129 g
p = 1 bar

if container valve opens shortly and 0.3 g of steam is led out what is pressure in the container after that?

Could yuo help how to address this problem

I have figured out that part of the water evaporates to steam. And it seems that PV=NRT equation can not be used?

Any help is appreciated

-Floid