Answer
2.8 kg
Work Step by Step
One mole of hydrogen gas= 2.0 g of hydrogen gas
occupies 22.4 L volume at STP.
$\implies$ grams of hydrogen needed to fill 31000 L=
$\frac{2.0\,g}{22.4\,L}\times31000\,L=2800\,g$
$=2.8\,kg$
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