Assuming an ideal gas: PV = nRT
Where P = pressure, V = volume, n = number of moles, R is the ideal gas constant, T is temperature
P = 1159 torr
V = 0.800 L
R = 62.364 L*torr/(mol*K)
T = 298.15 K
n is unknown, so solving for n gives us
n = PV/(RT) = 1159 * 0.800/(62.364*298.15) mol = 0.0499 moles of gas.
We know that this many moles weighs 1.40 g, so the molecular weight is 1.40 g/0.0499 mol = 28.1 g/mol.