Which one of the following is most acidic?

In the presence of moisture,$SO_2$ can

What is the product formed when $PCl_5$ reacts with concentrated $H_2SO_4$?

What is the number of allotropes of selenium?

Among the following allotropic forms of sulphur,the number of allotropic forms,which will show paramagnetism is ..... .
$(A)$ $\alpha$-sulphur
$(B)$ $\beta$-sulphur
$(C)$ $S_2$-form

Which of the following has $S-S$ bond?