\(PTK_{X_2O_5}=3,55.NTK_{Ca}=3,55.40=142\left(đ.v.C\right)\\ Mà:PTK_{X_2O_5}=2.NTK_X+80\left(đ.v.C\right)\\ \Rightarrow2NTK_X+80=142\\ \Leftrightarrow NTK_X=\dfrac{142-80}{2}=31\left(đ.v.C\right)\\ \Rightarrow X:Photpho\left(P=31\right)\\ \Rightarrow Hợp.chất:P_2O_5\)