Convert Kp to Kc?

  1. 1. The problem statement, all variables and given/known data

    At 25°C, Kp = 1x10-31 for the reaction below.

    N2(g) + O2(g) <--> 2 NO(g)
    (a) Calculate the concentration of NO (in molecules/cm3) that can exist in equilibrium in air at 25°C. In air PN2 = 0.8 atm and PO2 = 0.2 atm.

    2. Relevant equations

    3. The attempt at a solution
    I have calculated the partial pressure of NO to be 1.26x10-16 but I do not know how to convert to Kp from this. Any help would be greatly appreciated!
  2. jcsd
  3. Borek

    Staff: Mentor

    Sorry, but it doesn't make sense. Are you trying to convert Kp to Kc, or partial pressure to concentration, or something else?
  4. Gokul43201

    Gokul43201 11,141
    Staff Emeritus
    Science Advisor
    Gold Member

    First of all, you do not need to convert Kp to Kc.
    The number you have calculated for p(NO) is correct, though you are missing the units. What you need to do is convert partial pressure into molecules per cc.

    Hint: Ideal gas equation.
  5. I'm sorry - I am trying to convert partial pressure to concentration.

    Using the Ideal gas equation, PV=nRT, do I use n=2, solve for volume, and then convert to molecules per cm^3?
  6. Gokul43201

    Gokul43201 11,141
    Staff Emeritus
    Science Advisor
    Gold Member

    That's right.

    No, the stoichiometry doesn't matter anymore.

    Hint: You can find the molecules per cc if you first find the moles per unit volume, i.e., n/V
  7. I still am not getting the correct answer. I don't know what I'm missing.

    PV=nRT = 1.3E-16(V)=n(.08206)(298) so moles/L = 5.32E-18. I then converted it to moles/cm3 by dividing by 1000, which gave me 5.32E-21 then multiplied by Avogadro's number to get 3203.7 molecules/cm3.

    I also tried converting atmospheres to bars and using the gas constant 83.145 so that my answer would be in moles/cm3 and then converting from there, but that was not correct either.

    Can you tell me what I am doing wrong?
  8. Gokul43201

    Gokul43201 11,141
    Staff Emeritus
    Science Advisor
    Gold Member

    How do you know it's wrong? It looks good to me.
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