30. Solving a chemistry problem

170. To prepare a standard solution, a sample of high-purity aluminum weighing 0.0688 g was dissolved in hydrochloric acid and the solution was diluted with water to 1000.0 ml. Placed in volumetric flasks with a capacity of 50.0 ml: 1.00; 1.50; 2.00; 2.50; 3.00 ml of this solution, added sodium hydroxide to pH = 3.5, acetate buffer solution and xylene orange indicator. After heating and cooling, bring to the mark. To construct a calibration graph, we measured the optical densities relative to the first solution and obtained:
Vst, ml 1.50 2.00 2.50 3.00
Aotn. 0.075 0.150 0.225 0.280
A sample of apatite-nepheline ore weighing 0.1000 g after appropriate treatment was transferred to a 100.0 ml volumetric flask and the solution was brought to the mark. An aliquot of 1.00 ml was placed in a volumetric flask with a capacity of 50.0 ml, reagents were added as when constructing a calibration curve, brought to the mark with water and photometered under the same conditions. Calculate the mass fraction (%) of aluminum oxide.
option 170
Aotn.x 0.085