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It should not matter unless the question is asked in a way where only pressure or concentration is calculable. However, if either are acceptable, it is best to use pressure if the states of matter are all gas, and concentration if they are all aqueous.
Kp is used when you use partial pressures, which is for gases. Kc is normally used if you use molar concentrations in the equation, which is mostly for substances other than gases, however using the ideal gas equation (PV=nRT), you can convert pressure to concentration.
Normally, the question will specifically ask you to find either Kp or Kc. If the question is simply asking to write the expression for an equilibrium constant and doesn't give further direction, either Kp or Kc would be acceptable.
It becomes simpler when you see the phases that your reaction is in, which is usually given to you. For example, if it's in the gas phase you'd use partial pressures. Also if they are giving you units that relate to partial pressure then you'd use Kp.
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