PV = nRT
and n = mass / mw
PV = (mass / mw) RT
mw = mass RT / PV
P = 352 torr = 352 torr x (1 atm / 760 torr) = 0.463 atm
V = 0.206x10^-6 L
R = 0.0821 L atm /moleK
T = 45C = 45+273 = 318 K
mass = 243x10^-9 g
mw = (243x10^-9 g)x(0.0821 L atm /moleK)x(318 K) / [(0.463 atm)x(0.206x10^-6 L)]
mw = 66.5 g/mole