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OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012

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Original post by Killjoy-
You may want to use this equation:

pH=pKa+log[A][HA]\mathrm{pH}=\mathrm{pK_a} + \mathrm{log\frac{[A^{-}]}{[HA]}}

It is simply a logarithmic form of the Ka equation you are used to.

Remember [A]\mathrm{[A^{-}]} and [HA]\mathrm{[HA]} are the concentrations of conjugate base and undissociated acid at equilibrium.
So log[A][HA]\mathrm{log\frac{[A^{-}]}{[HA]}} represents the log to base 10 of conjugate base: acid ratio.

You need to find [A][HA]\frac{[A^{-}]}{[HA]} at equilibrium but in order to do this you need to find the pKa\mathrm{pK_a} value for the carbonic acid first.

1. The clue is to notice that you have been given the ratio at a particular pH, so this will enable you to find pKa\mathrm{pK_a} and use it to answer the question.

2. Once you have found pKa you sub it into the equation with the pH of blood the patient has and solve for the ratio.

I can't really type out all the working but I'll attach a picture:

Alternatively you may use the other form of the Ka equation, but that will involve more anti-logging I think.


thanks i sort of get it?

cheers
Reply 241
Original post by chemicalX
thanks i sort of get it?

cheers


Is it the maths involved that is the problem? Or the style of the question?
(edited 11 years ago)
Reply 242
Complex ion stereo isomers is confusing :s does anyone know how to draw them?
Original post by Killjoy-
Is it the maths involved that is the problem? Or the style of the question?


its the maths and the style of question LOL

im used to doing the buffer Q's with given Ka and then find out what they want normally without this ratio business.

Also i dont use the handerson haselbach method so that kind of confused me.

Can you possibly do it the standard(dum:colondollar:) way so i understand it.

Thanks
Reply 244
Original post by chemicalX
its the maths and the style of question LOL

im used to doing the buffer Q's with given Ka and then find out what they want normally without this ratio business.

Also i dont use the handerson haselbach method so that kind of confused me.

Can you possibly do it the standard(dum:colondollar:) way so i understand it.

Thanks


It's not a dum way! Sometimes one can be more efficient than the other. (In this case it turns out there isn't much of a difference.)
I'll post my method for the other way.

Spoiler

(edited 11 years ago)
Original post by Killjoy-
It's not a dum way! Sometimes one can be more efficient than the other. (In this case it turns out there isn't much of a difference.)
I'll post my method for the other way.

Spoiler


woow killjoy your writing is beautiful :smile:
Reply 246
Original post by otrivine
woow killjoy your writing is beautiful :smile:


Lol I think so too
Original post by Cath-ay
Lol I think so too


hi :wink: shall we revise? tomorrow morning ?
Reply 248
Original post by otrivine
hi :wink: shall we revise? tomorrow morning ?


Erm, okay. Anything topic in particular?
Reply 249
Original post by otrivine
woow killjoy your writing is beautiful :smile:


Thanks :redface:
Original post by Cath-ay
Erm, okay. Anything topic in particular?


CHAPTER 2 AND 1 ? or if you want we can do some chapter 1 and 2 now? :wink:
Original post by Killjoy-
Thanks :redface:


sorry if i sound a bit weird its cause your writing is neat and a pleasure to look at it while mine is like baby writing :colondollar:
do u want to revise/?
Reply 252
Original post by otrivine
sorry if i sound a bit weird its cause your writing is neat and a pleasure to look at it while mine is like baby writing :colondollar:
do u want to revise/?


That's ok. :smile:
Sorry, I can't today - my closest exam is FP3 on Friday and to prepare for that.
Reply 253
Original post by otrivine
CHAPTER 2 AND 1 ? or if you want we can do some chapter 1 and 2 now? :wink:


Explain how a CH3CHOHCOOH/CH3CHOHCOONa buffer system works. Include an equation and equilibrium. (5)
Original post by Cath-ay
Explain how a CH3CHOHCOOH/CH3CHOHCOONa buffer system works. Include an equation and equilibrium. (5)


CH3CHOHCOONA gives ACH3CHOHCOO- + NA+
CH3CHOHCOOH gives CH3CHOHCOO- + H+

equilibrium shifts to left to increase H+ ions as alkaline concentration increases
Equilibrium shifts to right to restore H+ ions as concentration of H+ ions increases
Reply 255
Original post by Cath-ay
Explain how a CH3CHOHCOOH/CH3CHOHCOONa buffer system works. Include an equation and equilibrium. (5)


Here's a nice question:
Why is is easier to make phenol into a buffer solution than benzene? (7 marks)
Reply 256
Original post by otrivine
CH3CHOHCOONA gives ACH3CHOHCOO- + NA+
CH3CHOHCOOH gives CH3CHOHCOO- + H+

equilibrium shifts to left to increase H+ ions as alkaline concentration increases
Equilibrium shifts to right to restore H+ ions as concentration of H+ ions increases


Erm I probably should have said to minimise pH changes. Sorry -.- the equilibrium stuff is along the right lines - mark schemes usually state which species reacts with the added acid or alkali.
And for the equation CH3CHOHCOOH <=> H+ + CH3CHOHCOO- would do :smile:
Original post by Cath-ay
Erm I probably should have said to minimise pH changes. Sorry -.- the equilibrium stuff is along the right lines - mark schemes usually state which species reacts with the added acid or alkali.
And for the equation CH3CHOHCOOH <=> H+ + CH3CHOHCOO- would do :smile:


so would i get full marks or not?
Reply 258
Original post by Dan12
Here's a nice question:
Why is is easier to make phenol into a buffer solution than benzene? (7 marks)


Is that from an actual paper?
Since when can you make a buffer solution from an alcohol :/ do tell...
Reply 259
Original post by Cath-ay
Is that from an actual paper?
Since when can you make a buffer solution from an alcohol :/ do tell...


I made it up, think about it if you add Na to phenol you'll form phenol and conjugate base (phenoxide). It's a synoptic question.

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