[Rutherford]

In the 5th problem, for part 3. and 4.: http://icho2014.hus.edu.vn/document/01-24-2014%20IChO46-Preparatory.pdf, what should be the units of Kp; atm, Pa, kPa, bar or maybe mmHg? I have no idea.

[Borek]

I would say: as long as you list the units in your answer, doesn't matter which you choose. And as the information is given using mmHg, I would go for mmHg just to make calculations easier.

[Rutherford]

The problem is that in previous calculations (using the plot) I obtain K_p2, but I don't know it's unit, while in the part 4. a reaction that has the constant K_p2 is given and the pressure of oxygen in mmHg. I don't know if the pressure of oxygen in mmHg can be used in the K_p2 expression, to calculate the partial pressures of other gasses. What if they thought that K_p2 calculated from the plot should be expressed in bars, but I use mmHg for the gasses' pressures? Won't give the correct answer.

[Borek]

The answer is... no idea.

Kp is not about pressures, just like Kc is not about concentrations. It is about unitless activities in both cases, which we just approximate by partial pressures and concentrations. To get the activity you should divide the pressure by a "standard" unit.

Trick is, it is not clear to me what the standard unit is. IUPAC suggests standard state to be 100 kPa (or 1 bar), but I believe for many years it was 1 atm, and it is still often used. Difference is not large, but it still exists.

I would go for bars, at least I know how to support it using IUPAC defined STP.

[Rutherford]

Would be nice if they uploaded the solutions, so I don't need to guess. Anyway, thank you for your help.

[Big-Daddy]

Borek already said this kind of but in case you haven't seen this I will try to clarify: for K, you have to divide all concentrations c by standard concentration c° and all partial pressures p by standard pressure p°. I'm pretty sure p° = 10⁵ Pa = 1 bar = 1/1.01325 atm etc. is the usual for IChO, it was last year as we discussed then.

e.g. if I have reaction N₂ (g) + 3 H₂ (g)  2 NH₃ (g), K = K_p (since these are all gases, activity is partial pressure) = (P(NH₃)²/P°²)/((P(H₂)³/P°³)*((P(N₂)/P°)) = (P(NH₃)² * P°²)/(P(H₂)³*P(N₂)). I'm sure you will see by definition K is unitless.

If you know this then I don't see where the confusion is. Standard pressure P° should be 1 bar on IChO.

[Rutherford]

Okay. I see. I can use even mmHg, using the standard pressure of 750mmHg/1.01325, but bars are better because the standard pressure is 1bar. Thank you.