Question

Conc. $H_{2}SO_{4}$ cannot be used for drying $H_{2}$, why?

Answer 1

A gas can be dried with sulfuric acid if the gas does not have a chemical effect with acid or reacts with sulfuric acid.

Hydrogen sulfide gas cannot be dried with sulfuric acid because ${ H}{ 2}S$ gas is oxidized to sulfur by sulfuric acid.  

Therefore, ${ H}{ 2}S$ gas has a chemical effect on acid and therefore, does not react with it.

${ H }_{ 2 }S{ { O }_{ 4 }\quad +\quad H }_{ 2 }S\quad \rightarrow \quad 2{ H }_{ 2 }O\quad +\quad S{ O }_{ 2 }\quad +\quad S$