Find the total numbers of compounds which contain $S-S$ linkage.

${\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_3,{\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_5,{\mathrm{H}}_2{\mathrm{S}}_4{\mathrm{O}}_6,{\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_7,{\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_8,{\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_6$

The compounds which contain $S-S$ linkage is ${\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_3,{\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_5,{\mathrm{H}}_2{\mathrm{S}}_4{\mathrm{O}}_6,{\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_6$ .Because S-S connection is present in the Sulphur oxides.

Hence the total numbers of compounds which contain $S-S$ linkage is