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.