calculate the volume of H2 gas kept at STP if it contain as many H atom as 1-98 gm H3 PO4 atomic mass of P is equal 31

Dear student,

The number of moles of H₃PO₄  =  1.98 / 98 =  0.02 moles 
Now the number of moles of H atoms = 3 x 0.02  =  0.06 moles
Again the number of H atoms in H₃PO₄ = 0.06 x 6.02 x 10²³  =  number of H atoms in H₂ gas 
Now the number of H₂ molecules = 1 / 2 x 0.06 x 6.02 x 10²³ 
Number of moles of H₂ gas  =  0.03 moles 
Now the volume of H₂ gas at STP  =  0.03 x 22.4 =  0.672 L
  Regards