29. Solving a chemistry problem

35a. To construct a calibration curve for the nephelometric determination of sulfate ion, 25.0 ml of a H2SO4 solution containing 0.215 mg/ml SO3 was placed in a 100 ml volumetric flask. Then in 100 ml volumetric flasks containing 20.0; 15.0; 10.0; 6.00 and 2.00 ml of this solution, BaSO4 suspensions were prepared and their apparent optical densities were measured:

V, H2SO4 ml 20.00 15.00 10.00 6.00 2.00
Dx,each 0.51 0.62 0.80 0.98 1.22

Based on these data, a calibration graph was constructed. The analyzed solution, 50.0 ml, was diluted in a 100 ml volumetric flask. Then 20.0 ml of this solution was transferred into a 100 ml volumetric flask. prepared a suspension of BaSO4 in it and brought it to the mark with water. The apparent optical density of this solution turned out to be:
Options a b c d
Dx,each 0.72 0.54 1.08 0.90
Determine the SO3 content in the solution (mg/l).