Convert Kp to Kc: N2 + O2 <--> 2NO @ 25°C

[needphyshelp]

Homework Statement

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.

Homework Equations

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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!}

 

[Borek]

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.}

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

 

[Gokul43201]

First of all, you do not need to convert Kp to Kc.

(a) Calculate the concentration of NO (in molecules/cm3) that can exist in equilibrium in air at 25°C.
...
I have calculated the partial pressure of NO to be 1.26x10^-16

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.

 

[needphyshelp]

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?

 

[Gokul43201]

I'm sorry - I am trying to convert partial pressure to concentration.

That's right.

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

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

 

[needphyshelp]

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?

 

[Gokul43201]

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