TPR Diagnostic 9 - PS #21

pm1

Full Member
Mar 21, 2012
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  1. Pre-Medical
    The passage talks about a container that has four different gases (He, O2, N2, CO2) at equal molar quantities.
    Then, one Experiment 1, the temperature of the compartment was gradually increased and the pressure was measured.

    As the temperature was increased in Experiment 1, which of the following would be expected to occur if the partial pressures of the four gases could be measured?
    A) The partial pressure due to helium would show the greatest increase.
    B) The partial pressure due to carbon dioxide would show the greatest increase.
    C) All of the partial pressures would show the same increase.
    D) None of the partial pressures would change.

    Answer: C

    I was really debating between C and D. I understand that the overall pressure increases since pressure is directly proportional to temperature. However, if the pressures from all respective gases are going to increase the same quantity wouldn't the partial pressure remain the same?

    Thanks! :)
     

    MedPR

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    1. Pre-Podiatry
      Partial pressure isn't a percentage. Partial pressure is how much pressure a specific thing contributes to the total pressure. So say the total pressure is 1atm before you heat it, then 5atm after you heat it. Even if all the gases increase by the same percentage, their partial pressures are all different.

      In the 1atm state they all had a pp of 0.25. Then at 5atm they all have a pp of 1.25.
       

      pm1

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      Mar 21, 2012
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        Partial pressure isn't a percentage. Partial pressure is how much pressure a specific thing contributes to the total pressure. So say the total pressure is 1atm before you heat it, then 5atm after you heat it. Even if all the gases increase by the same percentage, their partial pressures are all different.

        In the 1atm state they all had a pp of 0.25. Then at 5atm they all have a pp of 1.25.

        Right!! Thank you! I kept thinking it was percentage!
        And just to verify..
        Partial pressure is equivalent to the mole fraction of each gas, right? So since they are equimolar the partial pressure would show the same increase?
         

        Arctangent

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        Jun 10, 2011
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          Yes, partial pressure is proportional to the mole fraction, or as you said, they would show the same percentage increase. Also, in case this isn't clear, total pressure of a set of gases is equal to the sum of their partial pressures (Dalton's Law). So your reasoning that "overall pressure increases" would lead to choice C and eliminate choice D with Dalton's Law in mind.
           
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