
Is the pressure of H_{2} and O_{2}
the same when
electrolysis takes place? 
Question Date: 20130508   Answer 1:
When electrolysis takes place, the following
reaction occurs: 2H_{2}O → 2H_{2}
+ O_{2}. Twice as much hydrogen as oxygen
is produced. You can apply the ideal gas law,
PV=nRT, to deduce that at a constant pressure such
as 1 atmosphere you will produce twice the volume
of hydrogen as oxygen. If you are electrolyzing
water in an open environment, the gas produced
will exist at 1 atmosphere of pressure. The
pressure will be slightly higher if you use a
setup like this:
electrolysisapparatus
but that´s only because of the displacement of
water. In the case of that apparatus,
H_{2} would have a higher pressure because
more gas would be produced and therefore more
water would need to be displaced.
  Answer 2:
There is twice as much H_{2} produced
as O_{2} when water is electrolyzed.
Accordingly, if you had fixed volume containers to
collect the gases, you would expect the pressure
of H2 to be twice that of O_{2}. However,
in real life, we often operate under conditions of
fixed pressure, so you would expect the pressure
of H_{2} to be the same as O_{2},
but you'd get twice as much volume of H_{2}.
  Answer 3:
No  do the stoichiometry. There are twice as
many atoms of hydrogen in water as there are of
oxygen. There are the same number of atoms of
hydrogen in a molecule of elemental hydrogen as
there are atoms of oxygen in a molecule of
elemental oxygen. Therefore there will be twice as
many molecules of hydrogen as oxygen released by
electrolysis. Thus, the pressure of hydrogen will
be twice that of oxygen.
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