Let's take a quick look at what STP means
Standard Temperature (0 °C = 32 °F = 273.15 K)
&
Standard Pressure (1 atm)

We also know, that 1 mole of any gas at STP occupies 22.4 L

Now, we'll set up a ratio to find the moles of our gas

1 mole / x moles

=

22.4 L / 0.325 L

x = 0.0145089 moles


And finally

0.0145089 moles / 1 mole

=

0.805 g / y g

y = 55.48 g/mole


The molar mass of our gaseous compound is 55.48 g/mole