A spherical naphthalene ball of 2 mm diameter is subliming very slowly in stagnant air at 25°C. The change in the size of the ball during the sublimation can be neglected. The diffusivity of naphthalene in air at 25°C is 1.1×10^{-6} m²/s. The value of mass transfer coefficient is B × 10^{-3} m/s, where B (up to one decimal place) is _______.