Electron dot structure of ethyne is:-

A mixture of ethyne and air cannot be used for welding purpose because as we know ethyne is a unsaturated hydrocarbon so as when a unsaturated hydrocarbon is burnt in less amount of oxygen so it doesn't burns completely. Hence as air has just 21% of oxygen so when we burn ethyne in air, proper amount or amount necessary for welding is not produced. Hence mixture of ethyne and air is not used for welding purpose.
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