if you start with these 2 equations

.. PV = nRT... .. .... ... .. . <== ideal gas law

.. . .n = mass / mw.. .. . .<== definition of molar mass (abbreviated mw in my equation)

and substitute

.. PV = (mass / mw) * RT

rearranging

.. mw = (mass / V) * RT/P

and since density = mass / V

.. mw = density * RT/P

***********

so..

.. density = mass / V = 3.75g / 2.45L = ___ g/L

.. mw = (__ g/L) * (0.08206 Latm/molK) * (273.15K) / (1atm) = ___ g/mol

you get to finish