How to determine the vapor pressure of a volatile liquid?

 You take the initial minus final mass to determine how much benzene was saturated. 

7.7781-5.5139 = 2.2642 

Then divide by molar mass of benzene (78.11g/mol) to get the mols saturated. 

2.2642/78.11 = 0.02899 

Now plug into PV=nRT. After a small algebraic manipulation, we get P=nRT/V 

You are given everything else. Just plug in. Don't forget to change degrees Celsius to K. 
Your answer will be in atmospheres so you will need to multiply by 760 to get torr. 

Your answer should be: 106.8 torr (if I plugged in correctly)

You would do 1500/4 and the answer is 375

Solubility would be your answer.