A solar pond operates on the principle that heat losses from a shallow layer of water, which acts as a solar absorber, may be minimized by establishing a stable vertical salinity gradient in the water. In practice such condition may be achieved by applying a layer of pure salt to the bottom and adding an overlying layer of pure water. As a first approximation, the total mass density and the diffusion coefficient for salt in water (Dab) may be assumed to be constant, with Dab = 1.2e-9 m2/s. A saturated density = 380 kg/m3 is maintained for salt in solution at the bottom of the water layer of thickness L = 0.5 m. If the bottom is depleted of salt at the time that the salt density reaches 68% of saturation at the top, (a) determine the final (steady-state) density of the salt at the bottom. (b) Determine the time that the salt density reaches 68% of saturation at the top.