1. To determine phosphorus using the calibration graph method, a solution of 0.25 g of Na2HPO4 in 100 ml of water was prepared. The following volumes of this solution were taken and, after appropriate treatment, diluted with water to 25 ml. When photographing them, the following results were obtained:
Vst, ml 0.10 0.25 0.50 1.00 1.50
A 0.06 0.10 0.13 0.22 0.31
A 0.50 g sample of the metal was dissolved in a volume of 100, the optical density of the solution turned out to be 0.16. Determine the mass fraction of phosphorus in the sample (%).

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