b. A droplet of liquid 1, of radius r₁, is suspended in a stream of gas B. It is postulated that there is a spherical stagnant gas film of radius r₂ surrounding the droplet. The concentration of A in the gas phase is XA1 at r = r₁ and XA2 at the outer edge of the film, r = 72. i. By a shell balance, show that for steady-state diffusion r²NAr is a constant within the gas film. [6 marks] ii. By setting 2N equal to r2NAr1. the value at the droplet surface, derive the expression for the molar flux NA1 and further prove that the limit of the expression when r₂ → co is CDAB NAr1 = In 11 XB1- [9 marks] A water droplet, of radius 1.5 mm and temperature 293 K, is suspended in still air at a total pressure of 1.01325 x 105 Pa (1.0 atm). Water evaporates and diffuses through the air. By using the expression in part (a)(ii), determine the rate of evaporation at the surface of the droplet at steady state in kmol/s.m². Given: The vapor pressure and diffusivity of water vapor in air at 293 K and 1.0 atm pressure are 17.54 mmHg and 0.250 × 10-4 m²/s, respectively. The universal gas constant R = 8314.34 m³.Pa/kmol-K. [10 marks]