Vapours of alcohol are passed over hot copper to give ketone....

The cyclic ketal on hydrolysis with aqueous HCl yields acetone. This indicates that the ketone formed in the first step is acetone (CH_3COCH_3$).<br><br>When vapours of an alcohol are passed over hot copper at 573 K, secondary alcohols undergo dehydrogenation to form ketones. To form acetone, the starting alcohol must be a secondary alcohol with three carbon atoms.<br><br>Propan-2-ol (CH_3CH(OH)CH_3$) on passing over hot Cu undergoes dehydrogenation to give acetone.

Acetone reacts with ethylene glycol in the presence of dry HCl gas to form a cyclic ketal (ethylene glycol ketal of acetone).

Hydrolysis of this cyclic ketal with aqueous HCl regenerates acetone and ethylene glycol.

Therefore, the initial alcohol is propan-2-ol.

Answer: Propan-2-ol