The question asks only about pressure He and Dalton gave the answer many years ago. Dalton said that the partial pressure of a gas in a mixture of gases is independent of the other gases in the mixture. So the answer to the last question you have is that you don't need to worry about H2. You didn't change anything about He or Xe; therefore, the partial pressure of He (and Xe) is the same after adding H2 as it was before adding H2. The total pressure WILL change, it won't stay 7 atm because you add additional moles H2.
PV = nRT will give you T at the beginning. You can add the moles extra H2 and recalculate pressure and the total pressure will be greater than 7 atm. The pressure of H2 will change because more moles are present but the pressure of the other gases will not change.
Three gases, xenon, helium, and hydrogen are confined in a 10.0 L vessel at a pressure of 7.0 atm. Intially the vessel contains 3.0 moles of xenon, 2.0 moles of helium, and 2.0 moles of hydrogen. What would the partial pressure of helium be if 5.0 moles of hydrogen are added to the vessel? Assume that the volume and temperature of the vessel remain constant how do i start this? and how does it still equal 7atm if there are now 5 hydrogen? do i need to even worry about the hydrogen?
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