Answer :
There was 321.88 g of hydrogen before the reaction.
The equation of the reaction is;
P4 + 6H2 -------->4PH3
Number of moles of P4 = 123.9 g/124g/mol = 0.99 moles of P4
If 1 mole of P4 reacts with 6 moles of H2
0.99 moles of P4 reacts with x moles of H2
x = 6 moles × 0.99 moles
x = 5.94 moles of H2
Mass of H2 reacted = 5.94 moles of H2 x 2 g/mol
Mass of H2 reacted = 11.88 g of H2
Mass of H2 before the reaction = mass of hydrogen unreacted + mass of hydrogen reacted
Mass of H2 before the reaction = 11.88 g of H2 + 310 g of H2
= 321.88 g
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