4P + 5O2 = 2P2O5
P2O5 + 3 H2O = 2H3PO4
v(P) = 3,1 g/ 31g|mol = 0.1 mol
v(P) / 2 = V(P2O5) = 0.05 mol
v(P2O5) * 2 = v(H3PO4) = 0.1 mol
m(H3PO4) = 98*0.1 = 9.8 g
m(H2O)использованой = 0.15 mol * 18g|mol = 2.7 g
m(H2O) осталось = 150g- 2,7g = 147,3g
m раствора = 147,3g + 9,8g = 157,1 g
W(H3PO4) = (9.8g / 157.1g) * 100% = 6.2%