Which gas is used in metal welding?

In the reaction,$CH_3-C \equiv C-CH_3 \xrightarrow{(i) X, (ii) Zn/H_2O} CH_3-C(=O)-C(=O)-CH_3$,$X$ is:

The number of possible structural alkynes with molecular formula $C_5H_8$ is

Give a brief description of the structure of ethyne (acetylene).

Acetylene reacts with a $42\% H_2SO_4$ solution containing $1\% HgSO_4$ to give

One mole of $1, 2-$dibromopropane is reacted with $x$ moles of $NaNH_2$ followed by treatment with ethyl bromide to yield a pentyne. What is the value of $x$?