When to use K or Kc?

[906032986]

When I was doing textbook problem 5I.13, I got it wrong because I used the equilbrium constant K = 1.0 x 10^-5 instead of Kc = 1.2 x 10^-7. How do I know which one to use?

[Carissa Liu 3A]

I think in the textbook, K refers to Kp. If the units are in terms of partial pressures, then use K (Kp). If the units are in terms of molar concentrations, then use Kc.