Pure helium gas is stored at 20°C in the cylindrical Pyrex tank of 2 m in length with an inner diameter of 12 cm and a wall thickness of 2.5 mm. A helium detector situated beside the tank has detected a steady leakage of helium at 1.8 × 10^-6 g over the duration of an hour. Assume that both ends of the tank are impermeable surfaces, and the concentration of helium at the tank outer surface is negligible. Determine the concentration of helium at the inner surface of the tank.