Initially, a rigid 6-m³ tank contains helium (assumed to be an ideal gas) at 300 K and 0.10 MPa. The tank is then heated until the temperature reaches 400 K. A pressure-relief valve at the top of the tank opens when the pressure reaches 0.15 MPa, and helium flows out, preventing the pressure from ever exceeding 0.15 MPa. Determine the final mass (kg) of helium in the tank.