The following question is from Roorkee screening exam.I know the answer but I need an explanation on how the reaction proceeds.

Q.Dilute acetylene is passed through dil H2SO4 in the presence of HgSO4, the compound formed is Ethanal.

'x' gm of a compound A3B2C5 contains 'y' gm of A atoms

 

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how to represent the possible fischer projections for 2-bromohexane