CHEMISTRY COMMUNITY

The question asks for equilibrium partial pressures but we're given moles and volume. How do we get between these?

There's a conversion formula to convert Kp into Kc which would match the mole/L aspect of the problem. The formula is: Kp=Kc(RT)^delta n
Where R is the gas constant 0.08206 (L x atm)/ (mol X K)
T is the temp in Kelvins
delta n is the number of gaseous moles in product - number of gaseous moles in reactant

You do not have Kc however, you only need Kp. The question asks for partial pressures so you would convert your given mol of hydrogen gas into pressure. You are already given temp. Use PV=nRT and find the pressure of hydrogen gas. From there it should be straight forward. Hopefully this helps. Or someone corrects me.

The question does not give you moles... no wait it does in the equation...! I am a bit confused about this question as well.

Going to type the question out so people do not have to refer to the book if they answer/understand:
Bromine chloride, bromine, and chlorine gases have reached equilibrium in a container at 500 K according to the reaction,
2 BrCl(g) <--> Br2(g) + Cl2(g). Equilibrium partial pressures are 1.08 bar BrCl, 4.9 bar Br2, and 7.6 bar Cl2. Half the BrCl is removed fro the container and equilibrium is reestablished. Calculate the new partial pressure of BrCl in the new equilibrium mixture.

My question is: what do we do with the half BrCl that is removed? Do we cut the 1.08 bar in half... or do we change the coefficient 2 into 1? Or...?

I think you guys are talking about two different questions.
The question asked was from this year's practice quiz. You do not need Kc, you only need Kp. First you use P = nRT/V to find Ph2o. Then, draw an ice table to find an equation for Kp, which is given (3.72). Use the quadratic formula to solve for x, and you have the partial pressures.

How do you know which unit of pressure to use? Like how do we tell when to use atm, bar, etc.? And does it even matter?

Yes it does matter, but you do not know which unit to use because it is not given in the question. I'm pretty sure Lavelle forgot to put it in. I'm just writing down three different answers.

In the book in Example 10.8, when a specific unit was not given, and they converted from moles to pressure, they put it in bars, so that's what I'm going with. So Patrick, are your three answers in atm, bar, and Torr? And do you have significantly altered answers when for each set?

You get similar answers for atm and bar, since the constants are similar (8.31x10^-2 vs 8.21x10^-2), and if you round to 2 sig figs in your answer, you get the same answer for one, and .01 off for the other. But for Torr, the answer is way off.