Please help with Chemistry question *Guaranteed BEST ANSWER*?

Question

I've been thinking and rethinking this problem for about 30 minutes now. I thought it would be less time consuming to ask some one who knows what their doing for help lol If you could explain the right answer


A pure substance, above its melting point, is in a high pressure cylinder. UPon opening a valve on the cylinder a gas escapes. A pressure gauge on the cylinder shows a pressure of 56.5 atm, at 20 Celsius before opening the valve. After removing 10 ft cubed of the gas, measured at standard conditions, the pressure in the cylinder reads 56.5 atm at 20 Celsius. The pressure gauge is in good working order. WHich best explains these observations?

A. The cylinder and contents will weigh the same before and after opening the valve.
B. The substance in the cylinder is in the gaseous state.
C. The substance in the cylinder is mostly in the liquid state.
D. The substance in the cylinder has diatomic molecules when in the gaseous state.
E. The substance in the cylinder is oxygen.

your help would be MUCH appreciated.

Answer 1

C. The substance in the cylinder is mostly in the liquid state.

56.5 atm is simply the vapor pressure above the liquid. The liquid is maintaining it's vapor pressure despite being vented.