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### Difference between C and P?

Posted: Thu Jan 10, 2019 2:38 pm
When looking at equilibrium reactions where there are products and reactants in a gas state, how do I know when to use molar concentration (with square brackets in equilibrium constant equations) or the partial pressure (P in equilibrium constant equations)?

I understand that it often depends on the context of the equation, but I am working on question 11.13 (6th edition), and the question simply gives a reaction, so I am not sure which variable is more appropriate.

### Re: Difference between C and P?

Posted: Thu Jan 10, 2019 2:41 pm
I think that for part b, you would use the concentration because the products are aqueous solutions. Whereas if they were gases, you would use the partial pressure.

### Re: Difference between C and P?

Posted: Thu Jan 10, 2019 2:42 pm
Because part A) is in terms of gases, you would use the partial pressures of the gases.

### Re: Difference between C and P?

Posted: Thu Jan 10, 2019 2:42 pm
I would say to use Qp when the reactants and products are in gaseous state, and use Qc when the reactants and products are in aqueous state.

### Re: Difference between C and P?

Posted: Thu Jan 10, 2019 2:42 pm
If they just give the formula, I think it's safe to write the formula using partial pressure when gases are present, and use concentration brackets when aqueous compounds are present. However, if they give concentration numbers for gases, you can calculate K using those values.