4P+5O2\(\rightarrow\)2P2O5
P2O5+3H2O\(\rightarrow\)2H3PO4
nP=\(\frac{18.6}{31}\)=0,6(mol)
\(\rightarrow\)nP2O5=\(\frac{0,6}{2}\)=0,3(mol)
nH2O=\(\frac{21,6}{18}\)=1,2(mol)
\(\rightarrow\)H2O dư
\(\text{nH3PO4=0,3.2=0,6(mol)}\)
\(\text{mH3PO4=0,6.98=58,8 g}\)