84. Solving a chemistry problem

82. To determine free P2O5 in superphosphate, a 10 g sample of the latter was shaken with water, filtered into a 500 cm3 volumetric flask, washing the filter, brought the solution in the flask to the mark and mixed. When titrating 50 cm3 of the resulting H3PO4 solution (to NaHPO4), 16.51 cm3 of 0.1015 M sodium hydroxide solution was consumed. Calculate the mass fraction of P2O5 in superphosphate.