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