[student8607]

The partial pressures in an equilibrium mixture of NO, Cl₂ and NOCl at 500K are as follows:
0.240atm for NO
0.608atm for Cl₂
1.35atm for NOCl

I need to write the kp at 500K.

But how am I to do this without knowing the concentrations or the kc?

Kp=[PA]^a[PB]^b/[PC]^c
or
Kp=kc(RT)^{change in n}

[Astrokel]

Use partial pressure as the 'p' suggests.

Check this out: http://www.chemguide.co.uk/physical/equilibria/kp.html

[student8607]

http://www.chemguide.co.uk/physical/equilibria/kp.html
Kp has exactly the same format as Kc, except that partial pressures are used instead of concentrations.

I thought you had to have BOTH the partial pressures and the concentrations for Kp.

Thanks, that site was very helpful!

[Astrokel]

Nope, you only need partial pressures for K_p

[Loyal]

Yep one of the nicer things about gases.  Partial pressures and their mole fractions are interchangeable.