Question

why is the volume of gas collected at one electrode is double the other in electrolysis of water
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Answer 1

the volume of gas is collected at one electrode is double the other in the electrolysis of water because water contains hydrogen and oxygen and the two parts of hydrogen and 1 part of oxygen so the volume of hydrogen is double the volume of the oxygen so the volume of the gas is double the other

Answer 2

Composition of water is such that Hydrogen and Oxygen are in ratio 2:1

So volume of gas (hydrogen) collected at one electrode i.e. cathode is double the volume of gas at other electrode.

Hope this helps :)