135. Solving a chemistry problem

16. To construct a calibration graph for the determination of phosphorus in the form of a phosphomolybdenum complex, a solution was prepared by dissolving 0.25 g Na2HPO4 in 100 ml of water. The following volumes of this solution, after appropriate work-up, were diluted with water to 25 ml. Photometry of them yielded the following results:
Standard solution volume, ml 0.1 0.25 0.50 1.0 1.5
Optical density 0.06 0.10 0.13 0.22 0.31
A portion of the semiconductor metal 0.50 g was dissolved and after processing, 100 ml of a slightly colored solution was obtained. Its color was enhanced by adding 2 ml of the above standard solution; while the optical density was equal to 0.16. Determine the phosphorus content in the sample (in %)