Chemistry

I need help with how to find out the answers for questions 5,6 and 7. I put the link for the paper here: https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Topic-Qs/AQA/Physical-II/1.10-Equilibrium-Constant-Kp-for-Homogenous-Systems/Set-F/Equilibrium%20Constant%20(Multiple%20Choice)%20QP.pdf

Reply 1

Mabli.W

Original post by Idk1156

I've only just done this content myself but I had a go...

5. kp = (partial pressure of SO3)^2 / (partial pressure of SO2)^2 x (Partial pressure of O2)

That's from the formula for kp, the individual partial pressures have to be raised to the power=the number of moles in the stoichiometric equation
To find partial pressures we multiply the mole fraction by the total pressure in the vessel. The mole fraction is moles of the gas we care about as a fraction of the total moles of gas.

Total moles of gas = 0.05+0.07+0.08 =0.2

SO2: Mole fraction = moles of SO2/moles of all gases = 0.05/0.2 = 0.25
Partial pressure = mole fraction x total pressure = 0.25 x (540x10^3) = 135000Pa = 0.135MPa

Repeating that process for O2 and SO3 you get partial pressures of 0.216MPa and 0.189 MPa respectively

Putting all that into the equation for Kp gives 0.189^2 / 0.216 x 0.135^2 = 9.07

Reply 2

Mabli.W

6. To find out the units for an equilibrium constant you look at how many moles are on each side of the equation

Since Kp = products/reactants
Units of Kp = units of products/ units of reactants

The units of the products are Pa^2 because you will have used (pressure of SO3)^2
The units of the reactants are Pa^2 x Pa = Pa^3 from (Pressure of SO2)^2 x (Pressure of O2)

Pa^2/Pa^3 gives 1/Pa = Pa^-1
The question gives pressures in kilo pascals but that could be converted to megapascals like in the previous question

Reply 3

Mabli.W

7. I wasn't really sure how to do this with what i know about Kp/equilibria but it can be solved using the gas laws PV=nRT
Volume has to be converted from dm^3 to m^3 by dividing by 1000
Total moles = sum of moles of individual gases
Pressure given in question (should be converted to Pa rather than KPa)
R= 8.31 found on formula sheet

Substituting everything in gave me near enough 642K
Not sure if that's the method they wanted but I don't see another way XD