## Kp vs Kc

Jessica Chen 2C
Posts: 103
Joined: Thu Jul 11, 2019 12:17 am

### Kp vs Kc

How do we determine when we should use Kp versus when we should use Kc? I find myself defaulting to Kc, only to find that the answer is given in Kp.

Tyler Angtuaco 1G
Posts: 130
Joined: Wed Sep 11, 2019 12:16 am

### Re: Kp vs Kc

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.

Sarah Zhari 1D
Posts: 103
Joined: Sat Sep 14, 2019 12:16 am

### Re: Kp vs Kc

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.

asannajust_1J
Posts: 105
Joined: Wed Sep 11, 2019 12:16 am
Been upvoted: 1 time

### Re: Kp vs Kc

Use KP for gases with partial pressure values and use KC for concentration gases, and aqueous conditions.

ThomasNguyen_Dis1H
Posts: 102
Joined: Fri Aug 30, 2019 12:17 am

### Re: Kp vs Kc

Use Kp for gases and Kc for aqueous reactants and products.

Rodrigo2J
Posts: 102
Joined: Sat Jul 20, 2019 12:16 am

### Re: Kp vs Kc

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.

Jessica Esparza 2H
Posts: 130
Joined: Wed Sep 18, 2019 12:15 am

### Re: Kp vs Kc

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.