[cncbmb]

Source: 2007 U.S. Local Chemistry Olympiad

Water can be decomposed by the passage of an electric
current according to the equation

How many moles of can be produced from the
passage of electrons? Ignore the question mark. I don't understand why it is there.

http://www.physicsforums.com/showthread.php?t=234658
Borek responded to this problem with a question last year by asking "How many electrons needed per each H2 molecule?" You can see from the link above that there was no indication Asian Girl understood what Borek was pointing out. I don't understand either, except I don't even know why electrons would correspond to each H2 molecule, I don't see how to solve this in any of my chemistry textbooks, and I am having difficulty searching for this online.

How would we determine this how many electrons needed per each H2 molecule? Why would a certain number of electrons correspond to a mole of a substance? I don't see any electrons in the chemical equation.

[AWK]

2H⁺ + 2e^- = H₂

[cliverlong]

Hello,

   The piece of information missing from the question (which is probably intentional) is that the change from water to hydrogen and oxygen does does not happen directly.

   Initially, water self-ionizes into H₃0⁺ and OH^- ions

2 H₂O (l)  H₃O⁺ (aq) + OH⁻ (aq)

http://en.wikipedia.org/wiki/Self-ionization_of_water

  As these (electrically charged) ions are present (at low levels) an electric current can be carried in the liquid.

  Now to the question

How many moles of  H₂(g) can be produced from the passage of  4.8 x 10²¹ electrons?

Look at the equation for the discharge of the H₃O⁺ at the cathode to produce hydrogen (a more detailed version of what AWK has written)

2H₃O⁺ + 2e^- → H₂ + 2H₂O

What are the mole ratios in this equation?

How many moles is equivalent to  4.8 x 10²¹ electrons?

How can you combine this information to calculate the number of moles of H₂(g)

Clive

[Donaldson Tan]

Let's imagine the setup of how current is passed through water. You have a cathode and an anode immersed in a tank of water. The electric current flows from the anode to cathode via the water. The flow of electron is opposite of the conventional current.

So what happens?
At the cathode: 2H⁺ (aq) + 2e -> H₂ (g)
At the anode: 4OH^- (aq) -> 2H₂O (l) + O₂ (g) + 4e

2 electrons is needed to produce 1 molecular hydrogen

Moles of H₂ produced = (4.8*10^21) / [2*(6.023*10^23)] = 3.98*10^-3 moles

[cncbmb]

Thanks, everyone.
I understand now.