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# How much Hydrogen from a Gallon of Water?

07/25/2008 8:21 AM

Several of my coworkers and I got into a lively debate on how much hydrogen you can produce from a gallon of water if you were to pass electricity through it using electrodes. Does anyone out there have the calculations to calculate this?

As a starting point, we used a 24 V DC power supply and we used electrodes made out of stainless steel mesh.

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#1

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 8:30 AM
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#2

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 9:05 AM

In my 7TH grade chemistry, the volume difference was basically 2 to 1 - H20

H test tube was twice as the O test tube, have to look mass up

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#3

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 9:08 AM

The molecular weight of water = 18g/mol, and 2g of each 18g is hydrogen atoms, then the answer is 2/18 of the total starting material. 1 imperial gallon weighs 4.54kg or so, depending upon temperature, so there are 4.54kg x 2/18 = around 0.5kg of hydrogen to be had.

http://en.wikipedia.org/wiki/Electrolysis_of_water

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#4

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 9:22 AM

First principles

1 Ga of water weighs 3776.103 g

1 mol of water weighs 18 g

total mols = 209.7

2 H atoms = roughly 2 g/mol total

2*209.7 = 419.4 g/Gal of water.

The more interesting bit is the volume that this takes up.

PV = nrt

V = 209.7 * 8.314 * 290 / 101.325 (these numbers may be wrong, but they should be close...)

V = 5000 L

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#6

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 10:15 AM

I got about 523 liters when I did the calculation. Are you off by a factor of 10, or am I?

22.4 liters per mole at STP X 23.3 moles H2 per gallon of H2O = 523 liters.

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#9

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 10:35 AM

I had 209 mols in my calculations, not 23.3.

3776 g/Gal of water

take the total weight divided by 18g/mole

am i being stupid?

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#10

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 10:39 AM

I had 210 moles of water in 1 gallon -- maybe that's where you went wrong. There's only 23.3 moles H2 in 1 gallon of H2O.

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#11

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 10:57 AM

with further thinking...i am pretty sure you are wrong. It takes 210 moles of h2 and 105 moles of O2 to get 210 moles of h2o. I think what you are doing is trying to take a ratio of sorts, but that should only be used for weight...(i think).

2g h2/18 g of h2o = 11.111% of 3kg = 419 g of hydrogen.

2g/mol = M of H2

so

419/2 = 210 moles of h2.

210*22.4 = 4700 L

You are saying that in 3kg of water, there is only 46.6 g of h2 and i am pretty sure that is incorrect.

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#15

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 11:45 PM

You're right. Roughly 5 cubic meters of H2.

I trust that the original poster realizes that the energy value of those 5 cubic meters will be less that the energy value of the electricity used to do the electrolysis.

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#19

### Re: How much Hydrogen from a Gallon of Water?

07/26/2008 8:57 AM

The energy value of the hydrogen will be less than the energy expended on these calculations.

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#20

### Re: How much Hydrogen from a Gallon of Water?

07/26/2008 11:34 AM
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#21

### Re: How much Hydrogen from a Gallon of Water?

07/26/2008 12:21 PM

Substantially.

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#13

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 1:04 PM

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#5

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 9:50 AM

HI Everyone,

Thanks for the replies. I have enough information to be dangerous now.

Thanks for the info.

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#7

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 10:24 AM

Wow! Either 500 liters or 5000 liters that seems like a lot of hydrogen.

Once you have done this what is left over? Oxygen?

Let me know,

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#8

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 10:30 AM

Yes, oxygen is left over. H2 comes off the negative electrode, and O2 comes off the positive electrode. Of course, pure water is hard to hydrolyze, so adding a little salt is sometimes necessary, so it (whatever kind it may be) would be left over.

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#12

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 11:49 AM

did work for the phuemicutical industry, pure water is highly corrosive due to the reasoning of the ionization of the water. one would think that is nuetral PH, by theres some sort of reaction simuliar to an galvonic reaction going on

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#14

### Re: How much Hydrogen from a Gallon of Water?

07/25/2008 2:34 PM

'Tis true. I've been involved with chemists and biologists around the university here who have asked me to specify deionization systems for water.

Self-ionization of water is rather complex, but Wikipedia has a good treatise on the process.

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#16

### Re: How much Hydrogen from a Gallon of Water?

07/26/2008 12:26 AM

the answer is simple multiplication. 1800 gallons of hydrogen{mostly, but other volatile gases}in 1 quart of water, 4 quarts of water in 1 gallon 4x1800gallons=x hydrogen is also over 3 times as energy as gasoline per gallon.

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#17

### Re: How much Hydrogen from a Gallon of Water?

07/26/2008 1:13 AM

Huh?!

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#18

### Re: How much Hydrogen from a Gallon of Water?

07/26/2008 5:40 AM

You're comparing gallons of gas (a strange measure) against gallons of liquid. Use mass.

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