Verified answer

Partial Pressure

Of Hydrogen = Mole Fraction x Total Pressure

= moles of H2 / Total moles of mixture x 3 atm

= 2.5 / 3 x 3 atm

= 2.5 atm

The temperature is irrelevant. And the experimental description provided is dangerous -- the mixture might explode! Ignoring that, we see that the number of moles of H2 is 2.5, the number of moles of O2 is 0.5, so the total number of moles present is 3.0. Hence, the partial pressure of H2 is 2.5/3.0 x 3.00 atm, or 2.5 atm.