Quy ước: H(I), O(II)
CuO - Oxit
\(Đặt:Cu^aO^{II}\left(a:nguyên,dương\right)\\ \Rightarrow1.a=1.II\\ \Leftrightarrow a=\dfrac{1.II}{1}=II\\ \Rightarrow Cu\left(II\right)\)
HF - Axit
\(Đặt:H^IF^b\left(b:nguyên,dương\right)\\ \Rightarrow I.1=b.1\\ \Leftrightarrow b=\dfrac{I.1}{1}=I\\ \Rightarrow F\left(I\right)\)
SO3 - Oxit
\(Đặt:S^aO_3^{II}\left(a:nguyên,dương\right)\\ \Rightarrow a.1=II.3\\ \Leftrightarrow a=\dfrac{II.3}{1}=VI\\ \Rightarrow S\left(VI\right)\)
K2O - Oxit
\(Đặt:K_2^aO^{II}\left(a:nguyên,dương\right)\\ \Rightarrow a.2=II.1\\ \Leftrightarrow a=\dfrac{II.1}{2}=I\\ \Rightarrow K\left(I\right)\)
K2S - Muối
\(Đặt:K^a_2S^b\left(a,b:nguyên,dương\right)\\ \Rightarrow2.a=1.b\\ \Leftrightarrow\dfrac{a}{b}=\dfrac{1}{2}=\dfrac{I}{II}\\ \Rightarrow\left\{{}\begin{matrix}a=I\\b=II\end{matrix}\right.\\ \Rightarrow\left\{{}\begin{matrix}K\left(I\right)\\S\left(II\right)\end{matrix}\right.\)
CO2 - Oxit.
\(Đặt:C^aO^{II}_2\left(a:nguyên,dương\right)\\ \Rightarrow a.1=II.2\\ \Leftrightarrow a=\dfrac{II.2}{1}=IV\\ \Rightarrow C\left(IV\right)\)