A level chemistry question help needed please

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Bethanyjayne04
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Hi I've been given a worksheet of questions to complete but I'm completely stumped on the final question. Any help would be greatly appreciated .. this content hasn't been taught in class yet

KC for the equilibrium below 10.0 at temperature T. If 1.0 mole of the ester is mixed with 5.0 moles of water and the mixture allowed to reach equilibrium, how many moles of each species wilbe present at equilibrium ( you will need to solve a quadratic equation to answer this )
HCOOCH3(g) + H2O(g) - HCOOH(g) + CH3O(g)

Thanks in advance
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tony_dolby
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" this content hasn't been taught in class yet"

Are you sitting your A-level this year? If you are, it might be worth checking to see whether solving quadratic equations are required. Rest assured though, you can still score a lot of marks on an exam question like this without getting to the correct final answer.

Like any equilbrium question, write out the chemical equation. Underneath you put the moles present at the start of the reaction. In this case, 1.0 mole of ester and 5.0 moles of water. Zero moles for the acid and methanol. The molar ratios are all 1:1 so you can call the moles of acid 'x' and the moles of methanol 'x'.

You second line under the equation is moles present at equilibrium. The moles of water and ester will decrease by the value of 'x'. (5.0-x) and (1.0-x)

Write your expression for Kc. But remember that each term must be divided by V which is the volume. You'll get a mark for this.

Now, because there are equal numbers of moles on each side, the Vs will cancel out. You can write out the Kc expression again. Kc = x2 divided by (5.0-x) times (1.0-x)

You can now rearrange this fraction to the required form.
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Bethanyjayne04
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(Original post by tony_dolby)
" this content hasn't been taught in class yet"

Are you sitting your A-level this year? If you are, it might be worth checking to see whether solving quadratic equations are required. Rest assured though, you can still score a lot of marks on an exam question like this without getting to the correct final answer.

Like any equilbrium question, write out the chemical equation. Underneath you put the moles present at the start of the reaction. In this case, 1.0 mole of ester and 5.0 moles of water. Zero moles for the acid and methanol. The molar ratios are all 1:1 so you can call the moles of acid 'x' and the moles of methanol 'x'.

You second line under the equation is moles present at equilibrium. The moles of water and ester will decrease by the value of 'x'. (5.0-x) and (1.0-x)

Write your expression for Kc. But remember that each term must be divided by V which is the volume. You'll get a mark for this.

Now, because there are equal numbers of moles on each side, the Vs will cancel out. You can write out the Kc expression again. Kc = x2 divided by (5.0-x) times (1.0-x)

You can now rearrange this fraction to the required form.
Hi , .currently in my first year so won't be sitting my a levels untill next year.
Thanks for your help it's greatly appreciated, I'm struggling with format in which to set the question into.. I can do the workings out just not sure of how to lay it out on paper. Thanks tho
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Pigster
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(Original post by Bethanyjayne04)
Hi I've been given a worksheet of questions to complete but I'm completely stumped on the final question. Any help would be greatly appreciated .. this content hasn't been taught in class yet

KC for the equilibrium below 10.0 at temperature T. If 1.0 mole of the ester is mixed with 5.0 moles of water and the mixture allowed to reach equilibrium, how many moles of each species wilbe present at equilibrium ( you will need to solve a quadratic equation to answer this )
HCOOCH3(g) + H2O(g) - HCOOH(g) + CH3O(g)

Thanks in advance
Which spec. are you studying?
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Bethanyjayne04
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(Original post by Pigster)
Which spec. are you studying?
Aqa
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Cileen
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(Original post by Bethanyjayne04)
Hi I've been given a worksheet of questions to complete but I'm completely stumped on the final question. Any help would be greatly appreciated .. this content hasn't been taught in class yet

KC for the equilibrium below 10.0 at temperature T. If 1.0 mole of the ester is mixed with 5.0 moles of water and the mixture allowed to reach equilibrium, how many moles of each species wilbe present at equilibrium ( you will need to solve a quadratic equation to answer this )
HCOOCH3(g) + H2O(g) - HCOOH(g) + CH3O(g)

Thanks in advance
Okay so
HCOOCH3(g) +H2O -> HCOOH + CH3O
1mol 5mol X X
X-1 X-1 X+1 X+1
The only thing I don’t get is they didn’t tell you the equilibrium of any of those because you need an equilibrium to be able to solve for X that 10 equilibrium is for which element
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Pigster
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(Original post by Bethanyjayne04)
Aqa
I teach OCR A and it clearly states that the quadratic equation is not needed.

When I taught AQA, which was more than a decade ago, we didn't need to do quadratics. I would bet things haven't changed, but there is no reference to the quadratic equation on the spec. either to say that it is or isn't needed.
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APat1234
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(Original post by tony_dolby)
" this content hasn't been taught in class yet"

Are you sitting your A-level this year? If you are, it might be worth checking to see whether solving quadratic equations are required. Rest assured though, you can still score a lot of marks on an exam question like this without getting to the correct final answer.

Like any equilbrium question, write out the chemical equation. Underneath you put the moles present at the start of the reaction. In this case, 1.0 mole of ester and 5.0 moles of water. Zero moles for the acid and methanol. The molar ratios are all 1:1 so you can call the moles of acid 'x' and the moles of methanol 'x'.

You second line under the equation is moles present at equilibrium. The moles of water and ester will decrease by the value of 'x'. (5.0-x) and (1.0-x)

Write your expression for Kc. But remember that each term must be divided by V which is the volume. You'll get a mark for this.

Now, because there are equal numbers of moles on each side, the Vs will cancel out. You can write out the Kc expression again. Kc = x2 divided by (5.0-x) times (1.0-x)

You can now rearrange this fraction to the required form.
Yep, I'm pretty sure quadratics won't appear in exams
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tony_dolby
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(Original post by Bethanyjayne04)
Hi , .currently in my first year so won't be sitting my a levels untill next year.
Thanks for your help it's greatly appreciated, I'm struggling with format in which to set the question into.. I can do the workings out just not sure of how to lay it out on paper. Thanks tho
Learning how to lay out the solution neatly and logically is very important and helps eliminate silly errors. Jim Clark's Calculations in AS/A Chemistry is by far the best book around. In my opinion, Lister and Renshaw's AQA textbook has too many pictures and not enough substance.
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Bethanyjayne04
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Thanks everyone . Your help has been greatly appreciated
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