Agas sample with a mass of 12.8 g exerts a pressure of 1.2 atm at 15°c and a volume of 3.94 l. what is the molar mass of the gas?

We can use the ideal gas law equation to find the number of moles in the gas
PV = nRTwhere P - pressure - 1.2 atm x 101 325 Pa/atm = 121 590 Pa
V - volume - 3.94 x 10⁻³ m³
n - number of moles 
R - universal gas constant - 8.314 Jmol⁻¹K⁻¹
T - temperature - 15 °C + 273 = 288 K
substituting the values in the equation 
121 590 Pa x 3.94 x 10⁻³ m³ = n x 8.314 Jmol⁻¹K⁻¹ x 288 K
n = 0.200 mol
molar mass of gas is = mass / number of moles 
molar mass = 12.8 g / 0.200 mol = 64 g/mol 
molar mass of gas is 64 g/mol