дано

m(P) = 15.5  g

----------------------------

m(O2)-?

4P+5O2-->2P2O5

M(P) = 31 g/mol

n(P) = m/M = 15.5 / 31 = 0.5 mol

4n(P) = 5n(O2)

n(O2) = 5*0.5 / 4 = 0.625 mol

M(O2) =32 g/mol

m(O2) = n*M = 0.625 * 32 = 20 g

ответ 20 г