3.75 g of an unknown gas at 59 °C and 1.00 atm is stored in a 1.35-L flask. What is the molar mass of the gas?

You need to find moles of the gas, so you would use the ideal gas law:

PV=nRT

Pressure

Volume

n=moles

R = gas constant

Tenperature in Kelvin

n = PV/RT

(1.00atm) (1.35L) / (.08206) (332K) = 0.050mol

Molar mass is grams per mole, so

(3.75g/.050mol) = 75g/mol