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### How to know when to use partial pressure or molar concentrations?

Posted: Wed Jan 23, 2019 2:56 pm
If the problem doesn't specify whether to use partial pressure or molar concentrations, how do we know which to use?

For example, if a problem gives us the initial molar concentrations of the gaseous reactants, but doesn't specify whether to use partial pressures or concentrations, do we automatically assume to use molar concentrations, since the problem didn't give us any partial pressure values?

I seem to be missing something. Could someone please clarify? Thanks!

### Re: How to know when to use partial pressure or molar concentrations?

Posted: Wed Jan 23, 2019 3:25 pm
Hi! I think unless is specifies to use Kp or convert to partial pressures, if given the molar concentrations you can use those values with gases, as long as you are also using Kc.
I hope that helps!
-Fiona

### Re: How to know when to use partial pressure or molar concentrations?

Posted: Wed Jan 23, 2019 4:38 pm
If they give Kp, convert it to atm or bar, but if it gives Kc leave it in terms of the molar concentration.

### Re: How to know when to use partial pressure or molar concentrations?

Posted: Wed Jan 23, 2019 7:07 pm
You usually know when they ask for Kp (partial pressure) or Kc (molarity). You can also convert between the two by using the ideal gas law PV = nRT.

### Re: How to know when to use partial pressure or molar concentrations?

Posted: Thu Jan 24, 2019 12:05 am
When asked for Kp, use partial pressure and when asked for Kc, use molar concentrations. Partial pressure can be converted into molar concentrations and vice versa using the ideal gas law (Pv = nRT).