Acetamide on heating with phosphorus pentoxide gives

${\mathrm{CH}}_3{\mathrm{CONH}}_2\frac{{\mathrm{P}}_2{\mathrm{O}}_5}{-{\mathrm{H}}_2\mathrm{O}}⟶{\mathrm{CH}}_3\mathrm{CN}$

Phosphorus pentoxide has strong affinity for water and therefore, acts as a powerful dehydrating agent.

$\underset{\text{ acetamide }}{{\mathrm{CH}}_3{\mathrm{CONH}}_2}⟶\text{ anhydrous }{\mathrm{P}}_2{\mathrm{O}}_5⟶{\mathrm{CH}}_3\mathrm{CN}$

${\mathrm{CH}}_3\mathrm{CN}$ common name-Cyanomethane
IUPAC name-ethane nitrile.