Formula for relation between pressure and volume for GAS

Hi all,

I am brand new here and need some help - i hope it is the right category i am posting in.

I am no expert in physics - i am working at sea as a mate on board producttankers. I would like some advice for calculating or formulas for the following scenario:

- A closed tank 10m3 is half full with fluids(5m3)
- On top of the fluids there is pressurized inertgas (CO2); the pressure is 500mmWG absolute
- In the bottom of the tank there is a draining valve.

How many fluids can i drain before the pressure drops to Zero?

Thanks in advance.

Brgds, Stefan