\(PTK_{hc}=71.PTK_{H_2}=71.2=142\left(đ.v.C\right)\\ PTK_{hc}=PTK_{X_2O_5}=2.NTK_X+80\left(đ.v.C\right)\\ \Rightarrow2NTK_X+80=142\\ \Leftrightarrow NTK_X=\dfrac{142-80}{2}=31\left(đ.v.C\right)\\ \Rightarrow X:Photpho\left(KHHH:P\right)\Rightarrow A\)