Advanced Higher Chemistry 2009 Watch

teabreak
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#1
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Searched for this thread but there doesn't seem to be one yet =]

Post here with all your questions or just have a generally have a moan

I'll start:

"Calculate the pH of the buffer solution made by dissolving 2.4g of ethanoic acid and 1.64g of sodium ethanoate in water."

As I understand it, the equation for this is pH = pKa - log([acid]/[salt])

What I don't understand is how to get the concentration of the acid and the salt when all the information I have is the masses :confused:
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Achilles99
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I suppose that the volumes would cancel out anywho, so if you just put the moles in instead of concs. you would get the same answer- but I have never seen a q. without vol given before! :P
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MrCynical
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Moles = Mass/RMM, so calculate mole for each. Conc = moles/vol. Calculate that in terms of volume. You've now got [acid] = x/V and [salt] = y/V, where V= volume. Doesn't matter that you don't know V, because when you divide [acid] by [salt] in the pH calculation the Vs cancel out.
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Meteorshower
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MrCynical is spot on.
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teabreak
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Ahh that makes sense, thanks.
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Meteorshower
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Ok, I for one am starting to need this thread! Far too many trivial details I've forgotten :p:
My notes aren't that great either...

So, what is the weak oxidiser when it comes to alcohols and aldehydes... I can't for the life of me remember! (I know this is higher :p:)
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namedeprived
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Meteroshower, all I know is that primary alchhols are oxidies to aldehydes, which are further oxidised to alkanoic acids. Secondary alcohols are oxidised to form ketones.

Not sure if that helps you, not really sure what a weak oxidiser is, but based on the above I'd guess its the aldehyde.
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Meteorshower
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No, what I mean is something that oxidises an aldehyde but not a primary alcohol I think.
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namedeprived
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(Original post by Meteorshower)
No, what I mean is something that oxidises an aldehyde but not a primary alcohol I think.
Do you mean the oxidising agent? e.g Benedicts solution, acidified potassium dichromate solution.

If not I have no idea.:yep:
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Meteorshower
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(Original post by namedeprived)
Do you mean the oxidising agent? e.g Benedicts solution, acidified potassium dichromate solution.

If not I have no idea.:yep:
Yeah, that's what I mean :p: I think it might be benedict's that's the weak one... not sure though, don't have my higher notes any more.
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namedeprived
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(Original post by Meteorshower)
Yeah, that's what I mean :p: I think it might be benedict's that's the weak one... not sure though, don't have my higher notes any more.
Ah right I see. We haven't been tauht which of the oxidising agents are the weakest and which are the strongest, so I'm afraid I'm no help, sorry.
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teabreak
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Ok, 2005 MCq16:

Which of the following, when dissolved in distilled water, gives rise to a solution with a pH greater than 7?
A - Lithium chloride
B - Potassium ethanoate
C - Sodium sulphate
D - Ammonium nitrate

No idea how to do these types of questions, can anyone help me?
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abstraction98
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(Original post by teabreak)
Ok, 2005 MCq16:

Which of the following, when dissolved in distilled water, gives rise to a solution with a pH greater than 7?
A - Lithium chloride
B - Potassium ethanoate
C - Sodium sulphate
D - Ammonium nitrate

No idea how to do these types of questions, can anyone help me?
Which one comes from a strong base and a weak acid? That will give an alkaline salt and vice versa. So in this example its B.

Oh and Meteorshower. I have the opposite problem. Too many notes. A booklet for each unit each 70 pages long. Eurh
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abstraction98
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(Original post by Meteorshower)
Yeah, that's what I mean :p: I think it might be benedict's that's the weak one... not sure though, don't have my higher notes any more.
Standard one is acidified potatssium dichromate - goes orange to green. They are also big fans of:

1) Tollens Reagent (ammoniacal silver nitrate) - forms silver mirror

2) Cupper oxide is reduced to copper - goes black to brown.

3) Benedicts as you have mentioned - goes brick red.

Although I mainly come out with lithium aluminium hyridge (reducing agent) and acidified potassium dichromate (oxidising agent) unless it is specific.
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teabreak
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Damn, I knew that. Thanks =]
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Meteorshower
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(Original post by abstraction98)
Standard one is acidified potatssium dichromate - goes orange to green. They are also big fans of:

1) Tollens Reagent (ammoniacal silver nitrate) - forms silver mirror

2) Cupper oxide is reduced to copper - goes black to brown.

3) Benedicts as you have mentioned - goes brick red.

Although I mainly come out with lithium aluminium hyridge (reducing agent) and acidified potassium dichromate (oxidising agent) unless it is specific.
Thanks abstraction, but isn't one of the oxidising agents not as good as the others, and is only reduced when oxidising an aldehyde to a carboxylic acid?
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teabreak
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Anyone got any tips for remembering reagents? I don't think I've ever got one right!
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abstraction98
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(Original post by Meteorshower)
Thanks abstraction, but isn't one of the oxidising agents not as good as the others, and is only reduced when oxidising an aldehyde to a carboxylic acid?
Yeh well spotted. Acidified potassium dichromate is decent - it does everything. The rest don't as far as I'm aware. I'm 100% sure Tollens doesn't and I don't think Benedicts is powerful enoigh.

Copper oxide, I would need to check. If in doubt, acidified potassium dichromate :yep:
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Meteorshower
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(Original post by abstraction98)
Yeh well spotted. Acidified potassium dichromate is decent - it does everything. The rest don't as far as I'm aware. I'm 100% sure Tollens doesn't and I don't think Benedicts is powerful enoigh.

Copper oxide, I would need to check. If in doubt, acidified potassium dichromate :yep:
I think copper oxide does both, but i'm not 100%

Thanks for that though, i'll remember it now :cool:
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boysenberry
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In my revision I've ditched the LTScotland notes we got given, they're just too unwieldy to revise productively (they could be the same as yours, abstraction). Instead, Scholar's been my tool. The books are much smaller and concise. They make me happy, which is a miracle given the s**** that AH Chemistry is.
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