Answer:The first step is to calculate the moles of AgNO3 used:
0.0500 L x 0.082 mol/L = 0.0041 mol AgNO3
Since Ag3PO4 is formed in a 1:3 stoichiometric ratio with PO43-, the number of moles of PO43- is also 0.0041 mol.
The molar mass of Ag3PO4 is 418.6 g/mol:
0.0041 mol Ag3PO4 x 418.6 g/mol = 1.74 g Ag3PO4
Therefore, the percentage of P2O5 in the sample is:
(2 x 30.97 g/mol x 0.0041 mol / 4.258 g sample) x 100% = 30.0%
Note that the factor of 2 is used because P2O5 is the anhydride of H3PO4, and its molar mass is twice that of P.