Calculate the oxidation number of the underlined atom in the following molecules.
`H_(2)ul(C)_(2)O_(4),ul(C)_(6)H_(12)O_(6),ul(Pb)_(3)O_(4),ul(lF)_(7),Hul(Cl)O,ul(O)F_(2),ul(Ni)(CO)_(4),Hul(Au)Cl_(4),BaO_(2),Mg_(3)ul(N_(2)),ul(O_(3)),`

Explanation

To calculate the oxidation number of the underlined atoms in the given molecules, we will follow a systematic approach for each compound. Let's go through them one by one.

### 1. **Oxalic Acid (H₂C₂O₄)**
- Let the oxidation number of carbon be \( x \).
- The equation will be:
\[ 2(+1) + 2x + 4(-2) = 0 \]
...