A sphere of solid substance having a radius of 4 mm is suspended in a large volume of gas at 350 K and 0.98 bar. The percentage volume of that substance at the surface is 60% and at 4 mm around the substance is 5 %. The diffusivity of gas of that substance in the gas occupied it at 318 K, 420 K and 500 K is 2 × 10^-6 m²/s, 3 × 10^-6 m²/s, 5 × 10^-6 m²/s respectively. Calculate the molar rate of sublimation material from surface.