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You wish to measure the diffusion coefficient for a non-reactive gas, sulfur hexafluoride (SF_6). You place a closed chamber in contact with a porous material of interest and inject a small volume of SF_6 (with temporary venting so as to not change the air pressure) to create a concentration Co = 15 ppb and then measure the SF_6 concentration in the chamber at a few elapsed times: For the experimental set-up confining the gas to 1-D flow and a semi-infinite system (that is, the boundary on the right is not reached during the experiment), the SF_6 concentration in the headspace is given by the solution to the transient gas flow equation as C(0, t) = C_0 * e^beta * erfc(squareroot beta) where beta = aDt/L^2 In this expression, "a" is the air filled porosity (or volumetric air content), t is time, L is the length of the air chamber, and D is the diffusion coefficient of the gas (SF_6) in the porous media. Suppose the porous material is at a bulk density of 1.45 g/cm^3 and a volumetric water content of 0.12. Use the observed data and supplied information to estimate D, the diffusion coefficient for SF_6 in this porous media.