Help with question 11.7

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Kenan Kherallah 2C
Posts: 78
Joined: Fri Sep 28, 2018 12:17 am

Help with question 11.7

Postby Kenan Kherallah 2C » Wed Jan 09, 2019 9:42 pm

11.7 The following flasks show the dissociation of a diatomic molecule, X2, over time. (a) Which flask represents the point in time at which the reaction has reached equilibrium?
(b) What percentage of the X2 molecules have decomposed at equilibrium? (c) Assuming that the initial pressure of X2 was 0.10 bar, calculate the value of K for the decomposition.
I really need help with c. I dont get why the mole fraction formula gives you the correct answers? Can someone please explain.


Jerome Mercado 2J
Posts: 60
Joined: Tue Nov 28, 2017 3:02 am

Re: Help with question 11.7

Postby Jerome Mercado 2J » Wed Jan 09, 2019 11:38 pm

The fractions of the molecules is equivalent to to partial pressures of each molecule; just multiply those fractions by .10 to convert to partial pressures. Then use the equilibrium expression of partial pressures to get the equilibrium constant. Hope this helps.

Matthew Tran 1H
Posts: 165
Joined: Fri Sep 28, 2018 12:16 am

Re: Help with question 11.7

Postby Matthew Tran 1H » Thu Jan 10, 2019 7:46 pm

Pressure is directly proportional to the number of molecules/particles since all pressure really is is collisions of the particles against the container. So in this problem, there are 11 X2 molecules in the original container, so each molecule contributes a pressure of 0.10/11 bar per molecule. At equilibrium, there are 17 particles (5 X2, and 12 X from the decomposition of 6 X2), so therefore the total pressure is (17/11)*0.10 bar (size of the molecule doesn't matter when calculating pressure). You then could use fraction of X and X2 in the equilibrium container to find the partial pressures and calculate K. Hopefully you can see how you use both mole ratios and pressure ratios to get the answer!

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