Equilibrium Constant Question - Calculating initial mass from equilibrium moles Watch

Feraligatr
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:cookie:

Okay, A2 Chemistry question here from Edexcel Unit 4 June 2014 R Paper, Question 11.

According to the question, I have to calculate the initial mass from the equilibrium concentrations. The mark scheme for this question mentions that I have to convert equilibrium concentrations to moles for everything, calculate the total moles at equilibrium and then assume that the total moles at equilibrium will equal the initial moles of hydrogen iodide but I'm slightly puzzled. I'm pretty sure that the moles change at equilibrium right?

How though? Like just why does the total moles at equilibrium equal the initial moles of hydrogen iodide?

I was thinking that it might be due to the fact that hydrogen iodide is the only substance in the very beginning as suggested in the stem of the question? But I may be wrong though. Could anyone explain why though?
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B_9710
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What is the reaction?
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Feraligatr
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(Original post by B_9710)
What is the reaction?
Hydrogen Iodide(g) <--> Hydrogen(g) + Iodine(g)
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B_9710
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HI is the only substance in the beginning. If we multiply the conc. of I2 by the volume we get 1.44 x 10^-5 mol of I2 or H2.
If you look at the chemical equation you see that it is 2HI:I2
So if there is 1.44 x 10^-5 mol I2 then there is 2.88 x 10^-5 mol HI that has decomposed. Now work out the number of moles of HI that is in the equilibrium mixture and then you should be able to see how you can work out the original mass. - first find the original number of moles of HI
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B_9710
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The increase in number of moles of H2 or I2 is the half the number of moles that HI decreased by as it is at 2:1 ratio in the chemical equation
The number of moles of I2 or H2 before the reaction was 0. I have to assume this because you haver otherwise you can't work it out - unless the question states otherwise
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Feraligatr
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(Original post by B_9710)
HI is the only substance in the beginning. If we multiply the conc. of I2 by the volume we get 1.44 x 10^-5 mol of I2 or H2.
If you look at the chemical equation you see that it is 2HI:I2
So if there is 1.44 x 10^-5 mol I2 then there is 2.88 x 10^-5 mol HI that has decomposed. Now work out the number of moles of HI that is in the equilibrium mixture and then you should be able to see how you can work out the original mass. - first find the original number of moles of HI
Thank you :cookie:

So essentially the total moles at equilibrium will equal the initial moles of hydrogen iodide right?
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B_9710
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(Original post by Feraligatr)
Thank you :cookie:

So essentially the total moles at equilibrium will equal the initial moles of hydrogen iodide right?
That's correct
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Feraligatr
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That's correct
Thank you
I've repped you earlier :woo:
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B_9710
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(Original post by Feraligatr)
Thank you
I've repped you earlier :woo:
Thanks. Glad to help
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