Calculate the oxidation number of i) C in HCOONa ii) Cl in KClO3

i) Oxidation number of C in $\mathrm{HCOONa}$

1. Identify known oxidation numbers:

   • H = $+1$ (always in compounds except metal hydrides)
   • O = $-2$ (usually, except in peroxides)
   • Na = $+1$ (Group 1 metal)

2. Write the formula with oxidation numbers: $$\mathrm{H}{}^{+1}\mathrm{C}{}^{\times }\mathrm{O}{}^{-2}\mathrm{O}{}^{-2}\mathrm{Na}{}^{+1}$$

3. Set up the equation for neutrality: $$+1+x+2(-2)+(+1)=0$$ $$1+x-4+1=0$$

4. Solve for $x$: $$x-2=0⟹x=+2$$

Oxidation number of C in $\mathrm{HCOONa}$ = +2

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ii) Oxidation number of Cl in $\mathrm{KClO}{}_3$

1. Identify known oxidation numbers:

   • K = $+1$ (Group 1 metal)
   • O = $-2$ (usual oxidation number)

2. Write the formula with oxidation numbers: $$\mathrm{K}{}^{+1}\mathrm{Cl}{}^y\mathrm{O}{}^{-2}\mathrm{O}{}^{-2}\mathrm{O}{}^{-2}$$

3. Set up the equation for neutrality: $$+1+y+3(-2)=0$$ $$1+y-6=0$$

4. Solve for $y$: $$y-5=0⟹y=+5$$

Oxidation number of Cl in $\mathrm{KClO}{}_3$ = +5