Cho phản ứng hoá học: FeO + HNO₃ → Fe(NO₃)₃ + N_xO_y + H₂O. Hệ số cân bằng tối giản của HNO₃ là

$\overset{+2}{\mathop{Fe}}\,O\text{ }+\text{ }H\overset{+5}{\mathop{N}}\,{{O}_{3}}\to \text{ }\overset{+3}{\mathop{Fe}}\,{{\left( N{{O}_{3}} \right)}_{3}}+\text{ }{{\overset{+2y/x}{\mathop{N}}\,}_{x}}{{O}_{y}}+\text{ }{{H}_{2}}O$

$\begin{matrix}   \text{(5x-2y)}  \\   {}  \\   \text{1x}  \\\end{matrix}\left| \begin{align}  & \overset{+2}{\mathop{F\text{e}}}\,\,\,\to \,\,\overset{+3}{\mathop{F\text{e}}}\,\,\,+\,1\text{e} \\  & x\overset{+5}{\mathop{N}}\,\,\,+\,\,(5\text{x}-2y)\text{e}\,\,\to \,x\overset{+2y/x}{\mathop{N}}\,\, \\ \end{align} \right.$

(5x – 2y)FeO + (16x – 6y)HNO₃ → (5x – 2y)Fe(NO₃)₃ + N_xO_y + (8x – 3y)H₂O