Is a bottle of liquid hydrogen lighter when full or empty?

Answer 1

The real question here is whether liquid hydrogen is lighter than atmospheric air.

At normal pressure gaseous hydrogen is lighter than air. But liquids are more dense than gases. LOTS more dense. Water, for example, is 1244 times (!) more dense than water vapor. Most gases are fairly comparable. So without any calculations, I'd have to say the liquid is almost certainly heavier.

But for kicks, let's look up the numbers too. Liquid hydrogen at 20 Kelvin has a density of 70.8 kg/m^3. At sea level, Earth's atmosphere has a density of 1.2 kg/m^3. So our guess is proved... liquid hydrogen is 59 times more dense than the atmosphere, and thus the bottle is WAY lighter when empty than when full.

Answer 2

Assuming the hydrogen gas is at 14 PSI, the bottle of hydrogen gas would be lighter than the bottle of air (empty of gas).

However, most bottles of hydrogen gas are pressurized beyond 14 psi, so this answer is rather untrue in actuality....Pressurized hydrogen gas usually is found at something like 100 PSI or more.
So there is lots more of it than normal inside the bottle to make comparisons with.

If you wanted to make a real comparison and check on it for real,
compare two identical steel pressurized bottles - one with plain air (100 PSI) and one with hydrogen (100 PSI). you will find that the bottle of pressurized air weighs more.