A55.50 g gas sample of a gas, at stp, has a volume of 15.0 l. calculate the molecular mass of the gas

We can use the ideal gas law equation to find the number of moles of gas 
PV = nRT
where P - standard pressure - 101 325 Pa
V -volume - 15.0 x 10⁻³ m³
n - number of moles 
R - universal gas constant - 8.314 Jmol⁻¹K⁻¹
T - standard temperature  - 273 K
substituting the values in the equation 
101 325 Pa x 15.0 x 10⁻³ m³ = n x 8.314 Jmol⁻¹K⁻¹ x 273 K
n = 0.670 mol

molecular mass of the gas can be calculated as follows
molar mass = mass / number of moles 
                     = 55.50 g / 0.670 mol 
 molecular mass of gas is 82.8 g/mol