R is a liguid which reacts vigorously both concentrated aqueous ammonia to form a solid which is readily dehydrated by warming with phosphorous (V) oxide

The liquid 'R' reacts vigorously with concentrated aqueous ammonia to form a solid. This indicates that 'R' is an acylating agent, such as an acid anhydride or an acyl chloride, which reacts with ammonia (a nucleophile) to form an amide. Amides are typically solids at room temperature.

The solid product is readily dehydrated by warming with phosphorus(V) oxide ($\text{P}_4\text{O}_{10}$). Phosphorus(V) oxide is a strong dehydrating agent commonly used to convert amides into nitriles. This confirms that the solid formed in Step 1 is an amide.

Combining these observations, 'R' must be a liquid compound that reacts vigorously with ammonia to form an amide. A common example of such a compound is an acid anhydride. For instance, acetic anhydride ($\text{(CH}_3\text{CO)}_2\text{O}$) is a liquid that reacts with ammonia to form acetamide ($\text{CH}_3\text{CONH}_2$), which is a solid. Acetamide can then be dehydrated by $\text{P}_4\text{O}_{10}$ to form acetonitrile ($\text{CH}_3\text{CN}$).

A possible structure for the compound 'R' is acetic anhydride: