how to write the formula for K

[inlovewithchemistry]

I noticed in the solutions manual that when writing the equation for the K value, the concentrations were always written as the reactant or product as a subscript of P. Do we have to write it like that? I don't want to lose credit on my homework for that. Thank you!

[Kenan Kherallah 2C]

If they specify that they want Kp then you should write equilibrium constant in terms of partial pressures of the reactants and products. However, if you have substances that are not in the gaseous phase then you can't use Kp and must use Kc. If the book or the question doesn't specify whether Kc or Kp is wanted and the substances being considered are all gaseous, then it doesnt matter whether kp or kc is used.

[Nicole Lee 4E]

It depends on the state of the molecules in the equation. If they are gases then use P for partial pressure.

[Gillian Murphy 2C]

When it's written as a subscript of P, that means that it is using partial pressures instead of concentrations. If you are using concentrations, it's okay to write the brackets around it because that indicates that the unit is mol/L. Just double check to make sure which method you are actually using based off of what they give you.

[Lorena Zhang 4E]

If you are calculating Kp, then you need to write P subscript to indicate you are using the partial pressure. However, when you are calculating Kc, you can simply use the bracket to show the concentration.

[mbaker4E]

You only use subscript for P when you are representing partial pressure, which will only be used in chemical equilibrium problems with gas state molecules.

[juleschang16]

When it is written as a subscript of P, that indicates the partial pressure of a substance. You will only use this when the molecule is given in a gaseous state which is indicated by (g) as a subscript in a reaction. For example, PCl5(g) <--> PCl3(g) + Cl2(g)