39. A sample of phosphorus fertilizer weighing 2.500 g after appropriate treatment was diluted with water to 250.0 ml. A 100.0 ml portion of the solution was passed through a cation exchanger in the H+ form. The filtrate and washing waters were collected in a 500.0 ml volumetric flask. To titrate 100.0 ml of this solution with bromocresol green, 12.87 ml of 0.1 M NaOH (K = 1.017) were consumed:
HCl + NaOH = NaCl + H2O
H3PO4 + NaOH = NaH2PO4 + H2O
The same sample was titrated with a mixed indicator, using 24.85 ml of NaOH:
HCl + NaOH = NaCl + H2O
H3PO4 + 2NaOH = Na2HPO4 + 2H2O
Calculate the mass fraction (%) of P2O5 in the fertilizer. Answer: 43.24%.

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