Question

Hydrogen chloride gas being highly soluble in water is dried by:

• A. anhydrous calcium chloride
• B. phosphorous pentoxide
• C. quick lime
• D. concentrated sulphuric acid

Answer 1

The correct option is D

concentrated sulphuric acid

Hydrogen chloride gas being highly soluble in water is dried by concentrated sulphuric acid.
Explanation for the correct answer:
Sulphuric acid is used as a drying agent because it does not react with HCl.

Explanation for the wrong answer:
$\mathrm{HCl}$gas cannot be dried using drying agents like phosphorous pentoxide ${\mathrm{P}}_2{\mathrm{O}}_5$ and quick lime$\left(\mathrm{CaO}\right)$ because they react with HCl as follows:
$2{\mathrm{P}}_2{\mathrm{O}}_5\left(\mathrm{s}\right)+3\mathrm{HCl}\left(\mathrm{l}\right)\to {\mathrm{POCl}}_3\left(\mathrm{aq}\right)+3{\mathrm{HPO}}_3\left(\mathrm{aq}\right)$

$\mathrm{CaO}\left(\mathrm{s}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{CaCl}}_2\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

Thus, hydrogen chloride gas, being highly soluble in water, is dried by concentrated sulphuric acid.