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help please with this pH question? thanks

2008 9 pH.PNG


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answers
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could someone please explain?

i know the acid and sodium hydroxide react in 1:1 ratio but the question doesnt actually state the concentartion of each so I assumed the conc of each is equal but i dont know if you can do that

then I said if half of acid is neutralised that is 12.25 cm3 and assuming equal concentraion of acid and alkali that means this will happen when
12.25 cm3 of sodium hydroxde has been added so I went to 12.25 on x axis and read a pH of 4 of the graph which means pKa = 4

but the answers say
pKa = 3.75 - 3.8

what am I doing wrong?

thanks I really appreciate any help :smile:
exam on thursday ;(
2008 9 pH 2.PNG6.1KB
2008 9 pH 2 ans.PNG5.1KB
You have read the graph wrong. You can't assume the concs are the same and they are not as the equivalence point is when the curve goes vertical. Read that volume and that is the volume needed to neutralise the acid. You then take half that volume and read the pH at that volume.
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ah4p
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Original post by Madasahatter
You have read the graph wrong. You can't assume the concs are the same and they are not as the equivalence point is when the curve goes vertical. Read that volume and that is the volume needed to neutralise the acid. You then take half that volume and read the pH at that volume.


thank you :smile:

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