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

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