Helium gas at a pressure of 6.2 bar is stored in a fused silica container of spherical shape of 0.21 m diameter, the wall thickness being 2.6 mm. Calculate the mass of helium lost by diffusion. D = 0.04 × 10–12 m2/s. The solubility of helium in the material = 1.8 × 10–3 kg/m3 bar.