## Using partial pressure in Q

$E_{cell} = E_{cell}^{\circ}-\frac{RT}{nF}\ln Q$

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Tai Metzger 3K
Posts: 103
Joined: Sat Aug 24, 2019 12:17 am

### Using partial pressure in Q

Hi, I was working on a homework problem and in the question we are given concentrations for aqueous species involved in the reaction and the partial pressure of a gas that is a product of the reaction.

When setting up the Q expression, should we convert the partial pressure to concentration using PV=nRT? In other words, in the Q expression, is it okay for some of the species to be in M and some to be in barr? I had thought that they all should have the same units but in the answer key it is left in units of barr while the aqueous species are in M but this does not make sense to me.

Thanks!
Last edited by Tai Metzger 3K on Wed Mar 04, 2020 12:38 pm, edited 1 time in total.

Brianna Becerra 1B
Posts: 117
Joined: Fri Aug 02, 2019 12:16 am

### Re: Using partial pressure in Q

If partial pressure is used in both the reactant and product, then converting it using Pv=nRT would make it so the conversion would not make a difference as you would be essentially canceling the conversion out.

Tai Metzger 3K
Posts: 103
Joined: Sat Aug 24, 2019 12:17 am

### Re: Using partial pressure in Q

Brianna Becerra 1B wrote:If partial pressure is used in both the reactant and product, then converting it using Pv=nRT would make it so the conversion would not make a difference as you would be essentially canceling the conversion out.

Is it okay to use both concentration and partial pressure in the same Q expression?

Lauren Tanaka 1A
Posts: 109
Joined: Sat Aug 17, 2019 12:18 am

### Re: Using partial pressure in Q

Yes you can use partial pressures and molar concentrations in Q. This is because partial pressures can be used to calculate the Q value as well as the molar concentrations of the reactants and products.

Rohit Ghosh 4F
Posts: 99
Joined: Thu Jul 25, 2019 12:17 am

### Re: Using partial pressure in Q

Yes, Q and K can both be calculated either in terms of partial pressures of concentrations depending on the context of the problem.

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

### Re: Using partial pressure in Q

Yes it's fine to have different units as long as they cancel out. I think you would need to convert if you had a situation where three species were given in mol L^-1 and one given in bar.

kausalya_1k
Posts: 50
Joined: Wed Nov 14, 2018 12:23 am

### Re: Using partial pressure in Q

Just like K, Q can be calculated in terms of partial pressures or concentrations. Therefore there is no need to convert pressure to concentration (also because as someone pointed out earlier, using the PV=nRT will just cancel out the units anyways).

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