Consider the biosensor device shown in the figure in the next column. The biosensor is designed to measure the concentration of solute A in the well-mixed liquid phase. At the base of the device is an electrode of surface area 2.0 cm². The electrode is coated with an enzyme that catalyzes the reaction A †’ 2D. When solute A reacts to product D, product D is detected by the electrode, enabling for direct measurement of the flux of product D, which at steady state can be used to determine the concentration of A in the bulk liquid. The rate of reaction of A at the enzyme surface is rapid relative to the rate of diffusion of A down to the surface. Directly above the enzyme-coated electrode is a gel layer of 0.30 cm thickness that serves as a diffusion barrier for solute A and protects the enzyme. The gel layer is designed to make the flux of A down to the enzyme-coated surface diffusion limited. The effective diffusion coefficient of solute A in this gel layer is D_Ae= 4.0 × 10^-7cm²/s at 20°C. Above the gel layer is a well-mixed liquid containing a constant concentration of solute A, c'_Ao. The solubility of solute A in the liquid differs from the solubility of A in the gel layer. Specifically, the equilibrium solubility of A in the liquid layer (c'_A) is related to the solubility of A in the gel layer (c_A) by c'_A= K · c_A, with equilibrium partitioning constant K 0.8 cm³gel/cm³liquid. The process is considered very dilute, and the total molar concentration of the gel layer is unknown. The concentration of product D in the well mixed liquid is very small so that c_Do‰ˆ 0. At 20°C, the electrode measures that the formation of product D is equal to 3.6 × l0^-5mmole D/h. What is the concentration of solute A in the bulk well-mixed liquid phase, c'_Ao, in units of mmole/cm³? 

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Well-mixed d'Ao liquid (C'Ao is constant) CA Gel layer z = L = 0.3 cm Enzyme-coated electrode A → 2D Surface area = 2.0 cm2