# Can I mix pressures and concentrations when finding equilibrium constant? [duplicate]

In order to find the equilibrium constant (or reaction quotient) I know I can find the value as the products over reactants raised to coefficient exponents. But, in all of the problems I have had to date, I have had either concentrations of solutions or pressures of gases. Now I have a problem with both. Can I mix the two?

Here is the chemical reaction:

$$\ce{2 Al (s) + 6 H+ (aq) -> 2 Al3+ (aq) + 3 H2 (g)}$$

I am told that the concentrations of the aqueous solutions are $$\ce{[Al3+] = 9.3701}$$ M; $$\ce{[H+] = 4.6}$$ M; and hydrogen pressure is 0.0271 atm.

If I can "mix" concentrations and pressures, then my $$K_{eq}$$ or (Q) is:

$$Q = \frac{9.3701^2 * 0.0271^3}{4.6^6}$$

$$Q \approx 1.84 E{-7}$$

If I can't mix them then I need to do some kind of conversion between concentration and pressure. I'm not sure how to do that. A conversion might be based on $$K_p = K_c(RT)^{\Delta n}$$ but I don't have enough information to make a conversion using that formula.

So, the bottom line, can I mix concentrations and pressures and, if not, how do I make a conversion from one to the other?

• The values used are indicative of activity and so can be either concentration or pressure – Safdar Faisal Nov 7 '20 at 17:28
• Thank you. If you put it in an answer, I will mark it so. Though this is likely viewed as a "simple problem", I am sure I am not the only person who has ever been a bit confused by it. – Randall Blake Nov 7 '20 at 17:33