Calculate the volume of hydrogen gas at rtp liberated at the cathode in the electrolysis of acidified water by 18 moles of electrons. (One mole of any gas at rtp occupies a volume of 24 dm^3).
At the cathode the positive H+ ions accept an electron and are converted to H; as hydrogen is not stable as an individual atom, it immediately combines with another H atom to form the stable H2 molecule.
For the formation of each H2 molecule we need 2 electrons.
18 moles of electrons can help convert a quantity of H+ ions equal to 18/2 = 9 moles. The 9 moles of hydrogen gas that are formed due to the 18 moles of electrons occupy a volume equal to 9 times the volume occupied by a single mole of hydrogen gas.
As one mole of any gas at rtp occupies 24 dm^3, 9 moles of hydrogen occupies 9*24 = 216 dm^3 of volume.
The volume of hydrogen gas liberated at rtp due to 18 moles of electrons is 216 dm^3.
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