The final product formed when acetylene gas is passed through dilute sulphuric acid in presence of heavy metal cation at $60-65°\mathrm{C}$is

When acetylene gas is passed through dilute sulphuric acid at 60 -65 $°$C in presence of mercuric sulphate as catalyst, a water molecule adds up and finally acetaldehyde is formed. This is called hydration. Vinyl alcohol and acetaldehyde are tautomers.

$\mathrm{CH}\equiv \mathrm{CH}+\mathrm{HOH}\underset{60°\mathrm{C},1\%{\mathrm{H}}_2{\mathrm{SO}}_4}{\overset{30-40\%{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }}\underset{\mathrm{ethenol}}{\left[{\mathrm{CH}}_2=\mathrm{CHOH}\right]}+{\mathrm{CH}}_3-\mathrm{CHO}$