Answer:
The answer to your question is: Pentane (0.011 moles)
Explanation:
Data
V = 350 ml = 0.35 l
T = 67°C = 340 °K
P = 0.9 atm
mass = 0.79 g
R = 0.082 atm L/mol°K
Formula
            PV = nRT
            n = PV / RT
            n = (0.9)(0.35) / (0.082)(340)
            n = 0.315 / 27.88
           n = 0.0112
Now
MW acetone = 58g
MW ethanol = 46g
MW ethyl acetate = 88 g
MW pentane = 72 g
For acetone
         58 g ------------ 1 mole
         0.79 g --------  x
         x = 0.014 moles
For ethanol
        46g --------------- 1 mole
        0.79g  ------------ x
        x = 0.17 moles
For ethyl acetate
        88 g ------------- 1 mole
        0.79 g -----------  x
        x = 0.0089 moles
For pentane
       72 g -------------- 1 mole
       0.79 g ------------  x
       x = 0.011 moles
The substance is pentane