What is the volume of H2 gas at -273 degree celsius?

-273 degrees celsius is absolute 0, which cannot be achieved

theoretically, however, the answer to your question is 0 (as goes for any gas, but here's another mind boggler for you: it won't be a gas at absolute 0 either)

more complex explanation below:

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The ideal gas is only ideal, if it follows this mathematical rule (the ideal gas law).

Dividing it by the pressure, you get: Volume = Mass * Some Constant * Temperature / Pressure

If the Temperture is zero you get: Volume = 0 / Pressure = 0

So the volume is 0, if the temperture is 0.

The ideal gas exists only theoreticaly.

Logically the volume can't be 0 and therefor no gas is ideal.