You have the wrong formula for ammonium dodecaphophomolybdate. it should be (NH4)3[P(Mo3O10)4] or you could have written (NH4)3P.12MoO3 The molar mass of this canary yellow ppt is 1876.58 g/mol.
%P in sample = mass ppt*(molar mass P/molar mass (NH4)3[P(Mo3O10)4])/mass sample*100 = 1.1682(30.97/1876.58)/0.2711*100 = % P. Check the substitutions to make sure I didn't make a typo.
If you want %P2O5 then multiply what you get for %P x (molar mass P2O5/2P) = ?
Post your work if you get stuck.
1.4 Orthophosphate (PO4) is determined by weighing as ammonium phosphomolybdate. (NHA)PO4.12M003. Calculate the percent P in the sample and percent P₂Os if 1.1682 g precipitate were obtained from a 0.2711 g sample.
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