The intermediate compound formed when acetylene is hydrated in presence of dil ${\mathrm{H}}_2{\mathrm{SO}}_4$ and ${\mathrm{HgSO}}_4$ is

The given reaction is oxymercuration- demercuration.

${\mathrm{C}}_2{\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{HgSO}}_4/{\mathrm{aqH}}_2{\mathrm{SO}}_4}{\to }{\mathrm{CH}}_2=\mathrm{CH}-\mathrm{OH}\iff {\mathrm{CH}}_3\mathrm{CHO}$

Here, ${\mathrm{CH}}_2=\mathrm{CH}-\mathrm{OH}$ is the intermediate.

Thus, ethenol is the required intermediate in the above reaction.