Measurement of diffusivity by the poinbsource method (Fig. 18C.1)¹, we wish to design a flow system to utilize the results of Problem 18C.1 for the measure of D_AB. The approaching stream of pure B will be directed vertically upward, and the gas composition will be measured at several points along the z-axis. 

(a) Calculate the gas-injection rate WA in g-moles/s required to produce a mole fraction x_A ≈ 0.01 at a point 1 cm downstream of the source, in an ideal gaseous system at 1 atm and 800°C, if v₀ = 50 cm/s and D_AB ≈ 5 cm²/s. 

(b) What is the maximum permissible error in the radial position of the gas-sampling probe, if the measured composition X_A is to be within 1% of the centerline value?