The final acid obtained during the manufacture of $H_2SO_4$ by the contact process is

The molecular formula of pyrosulfuric acid is

How many total statements are correct in the following?
$(i)$ $SO_2$ - antichlor,preservative
$(ii)$ $O_3$ - Disinfectant
$(iii)$ $Cl_2$ - Antiseptic
$(iv)$ $O_2$ - used in oxy-acetylene welding
$(v)$ $PH_3$ - used in smoke screens
$(vi)$ $HNO_3$ - Pickling of stainless steel

In the presence of moisture,$SO_2$ can

When $SO_2$ is passed through acidified $K_2Cr_2O_7$ solution:

Which of the following formulas represents fuming sulfuric acid (oleum)?