What is the oxidation number of sulphur in ${\mathrm{SO}}_4^{2-}$?

Balance the following equation :

${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+{\mathrm{H}}^{+}+{\mathrm{S}}^{2-} \to {\mathrm{Cr}}^{3+}+\mathrm{S}+{\mathrm{H}}_2\mathrm{O}$

Balance the following equation : 

${\mathrm{MnO}}_4^{-}+{\mathrm{H}}^{+}+{\mathrm{Cl}}^{-} \to {\mathrm{Mn}}^{2+}+{\mathrm{Cl}}_2+{\mathrm{H}}_2\mathrm{O}$

Balance the following equation :

${\mathrm{MnO}}_4^{-}+{\mathrm{H}}^{+}+{\mathrm{Fe}}^{2+} \to {\mathrm{Mn}}^{2+}+{\mathrm{Fe}}^{3+}+{\mathrm{H}}_2\mathrm{O}$

Balance the following equation :

${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+{\mathrm{H}}^{+}+{\mathrm{Cl}}^{-} \to {\mathrm{Cr}}^{3+}+{\mathrm{Cl}}_2+{\mathrm{H}}_2\mathrm{O}$

Balance the following equation : 

${\mathrm{MnO}}_4^{-}+{\mathrm{H}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_2 \to {\mathrm{Mn}}^{2+}+{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$

Balance the following equation by ion-electron method.

${\mathrm{Fe}}^{3+}+{\mathrm{Sn}}^{2+} \to {\mathrm{Fe}}^{2+}+{\mathrm{Sn}}^{4+}$