### Kc vs. Kp

Posted:

**Tue Nov 28, 2017 10:51 pm**When we are asked to write the equilibrium expression K for a reaction and we are not told whether to write that equation with respect to concentration or partial pressures, how do we know which way to write the expression? In question 11.3 on the homework, I originally answered it in terms of molar concentration but then changed it when I saw the solution manual write the answer in terms of partial pressure.

I understand that gases are typically referred to with respect to their partial pressures, but must we always assume to write equilibrium expressions K in terms of partial pressure and not molar concentration? Are there ever cases where we write K in terms of concentration and not pressure?

I understand that gases are typically referred to with respect to their partial pressures, but must we always assume to write equilibrium expressions K in terms of partial pressure and not molar concentration? Are there ever cases where we write K in terms of concentration and not pressure?