How much Hydrogen can be extracted from a gallon of water? Is there any reason why a fuel cell powered aquatic vehicle (boat, submarine) could not supplement the Hydrogen supplies on-board with Hydrogen extracted from sea water, thus extending their range?

Asked by: Michael Ogilvie


The amount of hydrogen extracted from a gallon of water can be found very easily using the molecular weight of H20 (water), Hydrogen and Oxygen, along with mass conservation. the molecular weight of water is 2 H (molecular weight 1) + 1 Oxygen (Molecular weight 16) for a total of 18. And for every Molecule of water converted, we would get 2 Molecules of Hydrogen.

So, now the question is, how many molecules of water are there in a gallon of water? The density of water is 1g/(cm3) so in 1 gallon of water ( about 3.785 Liters or 3785 cm3) the mass of the water is, 3785g. 1 mole of 6.02x1023 molecules of water is equal has the mass in grams equal to the molecular weight or 18 grams per mole. so 3785 grams corresponds to about 1.265 x 1026 molecules of water.

Now, if every single one of those molecules were converted into Hydrogen we would get twice as much hydrogen as we had of water. or 2.53 x 1026 molecules of hydrogen. however since hydrogen is a diatomic molecule, meaning that the hydrogen that we talk about is H2, we would get 1.265 x 1026 molecules of hydrogen. at 1 atmospheric pressure and 273K, 1 mole of hydrogen fills approximately 22.4L of volume. so 1.265 x 1026 molecules or about 210 moles, would fill 4707 Liters of volume.

As for the reason that you cannot merely extract hydrogen to continuously power your aquatic vehicle, it is due to simple conservation of energy. Simply put, the energy you use to convert the water in the ocean into hydrogen would be at most equal to the energy that hydrogen could provide. Of course due to frictional losses, from things like resistance in your apparatus, you would actually end up using up more energy to get the hydrogen than you would gain from using it as a fuel source.

Answered by: David Seo, Physics Undergraduate Student, UCB, Berkeley



Science Quote

'If you think education is expensive, try ignorance.'

Derek Bok
Science Sidebar | Science Education Articles
10 Ways to Keep Your Kids Interested In Science

Young children are natural scientists: they ask questions, pick up sticks and bugs outside, and are curious about the world around them. But as they get a bit older, many kids gradually lose their interest in science. They might see it as just another task at school, something that doesn't apply to their lives. Of course nothing could be further from the truth, so here are ten ways you can remind your kids that science is everywhere. Most of these are fun for adults, too! Continue reading ...

Top Selling

Here are our physics & astronomy bestsellers:
Magnetic Levitator - Classic
12 inch Galileo Thermometer
Cricket, Locust, Beetle and Crab
Solar Radiometer
Weather Station 4M Kit
3D Magnetic Field Tube
Clean Water Science 4M Kit
Revolving Multi-Color Fiberoptic Light
Periodic Table of Elements Poster - Laminated
Solar Science 4M Kit


USC University of Southern California Dornsife College Physics and Astronomy Department McMaster University Physics and Astronomy Department