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Chemistry AS Calculate Kc

I have a question I just for some reason do not get it

For the reactions H2(g) +I2(g) --> 2HI(g), 0.200mol H2 and 0.400mol I2 are placed in a sealed container of volume 4.0dm3. At equilibrium, 0.140mol H2 remains.
a) Determine the equilibrium amounts of I2 and HI present
b) Calculate the value of Kc under these conditions
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Original post by FabStudent
I have a question I just for some reason do not get it

For the reactions H2(g) +I2(g) --> 2HI(g), 0.200mol H2 and 0.400mol I2 are placed in a sealed container of volume 4.0dm3. At equilibrium, 0.140mol H2 remains.
a) Determine the equilibrium amounts of I2 and HI present
b) Calculate the value of Kc under these conditions


Moved to Chemistry.

And it may help you to get more assistance if you include your working so far.

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Have you seen this example? It used the same reaction and similar workings out so should be helpful -
https://www.mytutor.co.uk/answers/577/A-Level/Chemistry/How+do+you+calculate+the+equilibrium+constant%252C+Kc%252C+of+a+reaction%253F


So first you need to make a table of the starting number of moles of hydrogen, iodine and HI, then the change in the number of moles of each as they reach equilibrium, and then the number of moles of each at equilibrium.

Start - 0.2 hydrogen, 0.4 iodine, 0 HI.

You know that hydrogen goes from 0.2 moles to 0.14 moles when equilibrium is reached, so the change for hydrogen is -0.06 moles.
From the reaction you can see that 1 mole of hydrogen reacts with 1 mole of iodine to form 2 moles of HI. So you also have a change of -0.06 moles for iodine (1:1) and a change of +0.12 moles for HI (1:2).

Moles at equilibrium: 0.14 hydrogen, 0.34 iodine, 0.12 HI

Use concentration = moles/volume to find the concentrations of each and then plug the numbers into the Kc formula.

I've been struggling with Kc so maybe try it yourself after reading the example and see if you get the same answer :smile:
Just use rice method and find equilibrium moles. When writing kc expression the right hand side of the equation goes on the top of the fraction and the left hand side on the bottom

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