Q: Partial Pressures and Concentration?

[Manya Bali 4E]

In number 37c in Ch. 14 from the 6th edition, the solution manual uses a formula for the reaction quotient that is, Q = P(H2)*P(CL2)/[H+]^2[Cl-]^2. I thought that Q (and K too) had to be only in terms of partial pressure or concentrations. Why is this formula acceptable?

For reference, the problem asks to determine the potential of this cell Pt(s)|Cl2(g, 250 Torr)|HCL(aq, 1.0M)||HCl (aq, .85M)|H2(g, 125 Torr)| Pt(s)

[705192887]

I believe that since they give one part in pressure and one in concentration, you can just use those values to find E=E°-(.025693V/n)ln(P/[M]).
This still gives a usable relationship between the products and reactants. One half as 1 torr pressure with different concentrations while the other half has 1 M and different torrs of pressure. Hope this helps.

[Manya Bali 4E]

Ok, in this case I understand since that's all that is given. However, when would you be able to differentiate between using Q with only concentrations or partial pressures and using Q with both? From previous chapters, we had to use the ideal gas law PV = nRT to convert from one to the other in order to make them all the same. Is that equation even applicable in instances with galvanic cells?

[Alexa_Henrie_1I]

I am also confused as to why that conversion doesn't apply to galvanic cells.

[Xingzheng Sun 2K]

For the calculation of Q and K, the equation is that Q = (The concentration x pressure of Products)/(The concentration x pressure of reactants) in the case there are two or more products or reactants which each contains both concentration and pressure.