\(Fe\left(OH\right)_3:\\
\%m_{Fe}=\dfrac{56}{107}.100\%=52,336\%\\
\%m_O=\dfrac{16.3}{107}.100\%=44,86\%\\
\%m_H=100\%-52,336\%-44,86\%=2,804\%\\
CuSO_4:\\
\%m_{Cu}=\dfrac{64}{160}.100\%=40\%\\
\%m_S=\dfrac{32}{160}.100\%=20\%\\
\%m_O=100\%-40\%-20\%=40\%\)
\(Ba\left(NO_3\right)_2:\\
\%m_{Ba}=\dfrac{137}{261}.100\%=52,49\%\\
\%m_N=\dfrac{14.2}{261}.100\%=10,728\%\\
\%m_O=100\%-52,49\%-10,728\%=36,782\%\)