Just How Did They Measure Hydrogen's Weight?

Gizmodo has an interesting post on Korean researchers having developed an unmanned plane that uses half-a-kilo of hydrogen to make whopping 10-hour flights. Unlike fossil fuels, Hydrogen packs an immense amount of energy and a half-kilo of the thing is energy equivalent to 1.3 kilograms of gasoline (Source).

My question is, how does one go about measuring the weight of Hydrogen? Being the lightest substance, it's not like you can fill a balloon with hydrogen and tape it to a weighing scale as that would show a negative weight. Moreover, just how negative the weight will be depends on the atmospheric pressure at the time of measurement. Vacuum measurement seems like a good alternative, but as soon as you fill a test tube (previously holding vacuum) with Hydrogen, it should become lighter than its original weight.

Googling for Hydrogen's weight measurement led to some 404s in the book search and more 404s in the actual results. I'm wondering if Google is trying to hide this particular information from the public. One of the results points to heating Methane until it breaks down (The Methane molecule consists of one atom of Carbon and four atoms of Hydrogen) to release Hydrogen. A difference in the weight of Methane used and of the Carbon left over should give the true weight of Hydrogen, given that one knows just how many molecules of Methane were there to start with. Even then, isn't every Hydrogen atom like a wing for the Carbon atom?

If you find yourself inclined to answer, leave a comment. For the sake of simplicity, let's not get into Avogadro's number and such jargon - not every reader may be able to follow.