[Ag+]3[PO4]3-
Ag3PO4(aq) ----> 3Ag^+(aq) + PO4^3-(aq)
In pure water, the molarity of PO4^3-, which is 1.76x10^-5 M, also represents the molar solubility of Ag3PO4, that means that max 1.76x10^-5 mol Ag3PO4 dissolves in 1L (1000 mL) of water. When the amount of water is halved to 500 mL, the solubility of Ag3PO4 is also halved. (8.8x10^-5 mol).
Ag3PO4(aq) ----> 3Ag^+(aq) + PO4^3-(aq)
8.8x10^-5 mol ..... 3x8.8x10^-5 mol = 2.64x10^-4 mol
[Ag^+] = n / V = 2.64x10^-4 mol / 0.500 L = 5.28x10^-4 M
