A 0.7818g sample containing phosphate (PO4 3-) was dissolved. The phosphate was precipitated as MgNH4PO4 by addition of aqueous Mg2+ followed by addition of aqueous ammonia. After filtration and washing, the precipitate was ignited at 1000 degrees C to give 0.1080g of Mg2P2O7 (222.553 g/mol).
Calculate the percent phosphate in the original sample.