Diffusion coefficients for vapors in air are determined by measuring the rate of evaporation of a liquid in a vertical glass tube. For a 0.2 cm diameter tube filled with n-heptane at 21 C, calculate the rate of descent of the liquid level when the meniscus is 1 cm from the top, taking the diffusivity value of 0.071 cm2/s. At 21 'C the vapor pressure and density of n-heptane are 0.050 atm and 0.66 g/cm3, respectively