monkeyvonban
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http://www.edexcel.com/migrationdocu...e_20120613.pdf

Question 14 B (ii)
"*(ii) Explain how this buffer solution resists a change in pH when a few drops of sodium hydroxide are added." 3 marks

It is a buffer of ethanoic acid and sodium ethanoate

http://www.edexcel.com/migrationdocu...s_20120816.pdf


The third mark is for saying that the ratio of [CH3COOH] to [CH3COO-] remains fairly constant and IGNORE concentration of Hydrogen ions remains constant, why would that be ignored?

Any help will be greatly appreciated
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charco
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(Original post by monkeyvonban)
http://www.edexcel.com/migrationdocu...e_20120613.pdf

Question 14 B (ii)
"*(ii) Explain how this buffer solution resists a change in pH when a few drops of sodium hydroxide are added." 3 marks

It is a buffer of ethanoic acid and sodium ethanoate

http://www.edexcel.com/migrationdocu...s_20120816.pdf


The third mark is for saying that the ratio of [CH3COOH] to [CH3COO-] remains fairly constant and IGNORE concentration of Hydrogen ions remains constant, why would that be ignored?

Any help will be greatly appreciated
because it's implicit in the fact that the pH does not hardly change ...
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monkeyvonban
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(Original post by charco)
because it's implicit in the fact that the pH does not hardly change ...
From the equation

pKa=pH-log([CH3COO-]/[CH3COOH])

as pKa cannot change we have

log([CH3COO-]/[CH3COOH]) + a constant = pH

If we say [H+] doesn't change, that implies that the ratio remains constant, does it not?
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Borek
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Why does pH of the buffer doesn't change? Because its pH doesn't change.

Would you accept this answer? That's basically what saying "concentration of Hydrogen ions remains constant" means in the context of the question.
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monkeyvonban
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(Original post by Borek)
Why does pH of the buffer doesn't change? Because its pH doesn't change.

Would you accept this answer? That's basically what saying "concentration of Hydrogen ions remains constant" means in the context of the question.
But from the equation pKa=pH-log(A-/HA) saying the ratio doesn't change means exactly the same thing, as pKa is constant, so how is that any better?
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Borek
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Small change in the ratio is a CAUSE, small change of pH in the EFFECT. Question mentions the EFFECT and asks for CAUSE. Listing EFFECT as CAUSE won't work.
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monkeyvonban
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(Original post by Borek)
Small change in the ratio is a CAUSE, small change of pH in the EFFECT. Question mentions the EFFECT and asks for CAUSE. Listing EFFECT as CAUSE won't work.
Oh right, that does make sense.

Nice one mate
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