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# OCR A 2016 Chemistry A* A-Level Resources watch

1. As part of an investigation, a student needed to prepare a buffer solution with a pH value of8.71. From the Ka value of phenol, the student thought that a mixture of phenol and sodiumphenoxide could be used to prepare this buffer solution.The student decided to use a 0.200 mol dm–3 solution of phenol, mixed with an equalvolume of sodium phenoxide.Use your knowledge of buffer solutions to determine the concentration of sodium phenoxidesolution that the student would need to mix with the 0.200 mol dm–3 phenol solution ka=1.3*10^-10

i keep getting 0.013 but answer is 0.13
1.3*10^-10 *0.2/10^-8.71=0.013
2. (Original post by Saywhatyoumean)
Guys with the copper iodine equation, it says two of the iodide ions do not change oxidation state?

I was wondering how this fits into the half equations?

So I thought it would be:

Cu2+ + e- --> Cu+

2I- --> I2 + 2e-

But I know 2 iodide ions are missing from the overall equation so where would we put them? Or is it a matter of just balancing each side? Attachment 512427

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You need to balance both sides and then I think just add in the remaining iodide ions because they don't change oxidation state they are not involved in an ionic equation.
If someone could explain it better would be great!

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3. (Original post by Saywhatyoumean)
Guys with the copper iodine equation, it says two of the iodide ions do not change oxidation state?

I was wondering how this fits into the half equations?

So I thought it would be:

Cu2+ + e- --> Cu+

2I- --> I2 + 2e-

But I know 2 iodide ions are missing from the overall equation so where would we put them? Or is it a matter of just balancing each side? Attachment 512427

Posted from TSR Mobile
I didn't really get this either when I looked at it but I think it might be because the iodide ions are in excess?? So two of them change oxidation state to become I2 and then two iodide ions are needed to react with the 2 Cu2+ to form 2 CuI2?? Sorry that was a really crap explanation but I think if you just remember that two extra Iodide ions are required to react with copper (II) ions to form copper iodide you'll be fine

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4. (Original post by LThomas694)
I didn't really get this either when I looked at it but I think it might be because the iodide ions are in excess?? So two of them change oxidation state to become I2 and then two iodide ions are needed to react with the 2 Cu2+ to form 2 CuI2?? Sorry that was a really crap explanation but I think if you just remember that two extra Iodide ions are required to react with copper (II) ions to form copper iodide you'll be fine

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Oh okay, that makes a bit more sense - also assuming you mean CuI?

I'm hoping they won't ask us for half equations then lol I don't know what I would put...

Maybe just add an extra equation for 2I- + 2Cu+ --> 2CuI

But thanks

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You need to balance both sides and then I think just add in the remaining iodide ions because they don't change oxidation state they are not involved in an ionic equation.
If someone could explain it better would be great!

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Yeah that's what I thought aha thanks, still not quite sure of the half equations we'd need but maybe they won't ask us for those...or they'll accept the ones I put? because technically that's what's happening right, if we ignore the Cu lol

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6. (Original post by Saywhatyoumean)
Yeah that's what I thought aha thanks, still not quite sure of the half equations we'd need but maybe they won't ask us for those...or they'll accept the ones I put? because technically that's what's happening right, if we ignore the Cu lol

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Thinking about it as I2 is a solid they will probably say that a precipitate is formed which will hunt at the l2.

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Thinking about it as I2 is a solid they will probably say that a precipitate is formed which will hunt at the l2.

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But what would that hint to exactly?

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8. (Original post by R.abokor)
As part of an investigation, a student needed to prepare a buffer solution with a pH value of8.71. From the Ka value of phenol, the student thought that a mixture of phenol and sodiumphenoxide could be used to prepare this buffer solution.The student decided to use a 0.200 mol dm–3 solution of phenol, mixed with an equalvolume of sodium phenoxide.Use your knowledge of buffer solutions to determine the concentration of sodium phenoxidesolution that the student would need to mix with the 0.200 mol dm–3 phenol solution ka=1.3*10^-10

i keep getting 0.013 but answer is 0.13
1.3*10^-10 *0.2/10^-8.71=0.013
Where is this question from? I got the same answer as you too I don't see what you did wrong, maybe there's a mistake in the question?

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9. (Original post by Saywhatyoumean)
But what would that hint to exactly?

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Suggested question.
Copper and iodine are mixed forming a solution and a solid.
Work out the full equation.

That sort of hint ???

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Suggested question.
Copper and iodine are mixed forming a solution and a solid.
Work out the full equation.

That sort of hint ???

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is that the full question?

Good work helping each other guys! In most cases people seem to answer each other before I have a chance to even see the question! I'll get round to it eventually just post it again if I miss it.

I'm not going to work out what the question is, please post the full question if you want me to answer

Here's a video that explains it well I think

11. (Original post by ForgottenApple)
is that the full question?

Good work helping each other guys! In most cases people seem to answer each other before I have a chance to even see the question! I'll get round to it eventually just post it again if I miss it.

I'm not going to work out what the question is, please post the full question if you want me to answer

Here's a video that explains it well I think

Hey no it's not the full question, it's actually not a question at all I was just wondering if they would ask us for the half equations of the reaction and what we'd be expected to write as they don't balance to form the overall equation

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12. (Original post by Saywhatyoumean)
Hey no it's not the full question, it's actually not a question at all I was just wondering if they would ask us for the half equations of the reaction and what we'd be expected to write as they don't balance to form the overall equation

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Each side of Half equations need to balance. The sum of some multiples of all the half equations must sum to the overall equation.
13. How many past papers did you do to get your A*? And how many times did you do each?

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14. (Original post by UnknownAnon)
How many past papers did you do to get your A*? And how many times did you do each?

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Mate you don't even want to know
15. (Original post by ForgottenApple)
Mate you don't even want to know
You're right. I don't
But I feel like I have to know

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16. This is absolutely amazing, thank you so much! You're a star.
17. (Original post by UnknownAnon)
You're right. I don't
But I feel like I have to know

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I mean I actually compiled statistics for my A levels. Everyone needs a hobby.
18. Has done the investigating the properties of transition metals qualitative yet? I've got it in a few weeks and I screwed up the last qualitative on TM elements and we're only doing two qualitatives so I can't mess this one up too! I was just wondering how hard it is in comparison to the other TM qualitative and what's best to revise/watch out for? Thank you

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19. (Original post by LThomas694)
Has done the investigating the properties of transition metals qualitative yet? I've got it in a few weeks and I screwed up the last qualitative on TM elements and we're only doing two qualitatives so I can't mess this one up too! I was just wondering how hard it is in comparison to the other TM qualitative and what's best to revise/watch out for? Thank you

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Easier said than done but seriously... write down what you observe

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20. (Original post by Hunnybeebee)
Easier said than done but seriously... write down what you observe

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Alright thank you I guess that's all you can do eh, I'm pretty sure I dropped 3 marks last time because my precipitate got stuck to the side of the tube and didn't dissolve... just gotta make sure that doesn't happen again ahaha

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