equivalence point

[mikemohan0482]

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Doing a Gold Standard practice test I came across a physical sciences question that said "at the equivalence point of a titration, which of the following situations exists?" and the correct answer was "[H+] = [OH-]"

Now this I don't get. If [H+]=[OH-], the pH=7, right? How can pH always equal 7 at the equivalence point? what if you've got acetic acid and the pKa is 8 or 9. At pH=8 or 9 H+ will not equal OH- or it wouldn't be 8 or 9. What gives? I'm kicking ass in physical sciences except for some idiotic reason this titration crap. Lend a hand? How was that answer right?

 

[daffy]

Originally posted by mikemohan0482
Doing a Gold Standard practice test I came across a physical sciences question that said "at the equivalence point of a titration, which of the following situations exists?" and the correct answer was "[H+] = [OH-]"

Now this I don't get. If [H+]=[OH-], the pH=7, right? How can pH always equal 7 at the equivalence point? what if you've got acetic acid and the pKa is 8 or 9. At pH=8 or 9 H+ will not equal OH- or it wouldn't be 8 or 9. What gives? I'm kicking ass in physical sciences except for some idiotic reason this titration crap. Lend a hand? How was that answer right?

PH is always 7 @ eq point if you have strong A w/ strong B, this must've been the case

 

[zinjanthropus]

try and remember that the equivalence point is when there is just as much H+ floating around as OH- (just as much titrant and conjugate).....H+ and OH- are equivalent....that's what the equivalent point is....now, that can occur at slightly different levels depending on whether you are titrating with a acid or base (just above or just below 7)

 

[mikemohan0482]

Originally posted by daffy
PH is always 7 @ eq point if you have strong A w/ strong B, this must've been the case

that's what i figure. Only they did not specify, so I'm taking it as a flawed question. especially since the answers [H+] = 10-7 and [oh-] = 10-7 were wrong.

 

[daffy]

Originally posted by mikemohan0482
that's what i figure. Only they did not specify, so I'm taking it as a flawed question. especially since the answers [H+] = 10-7 and [oh-] = 10-7 were wrong.

did you have the graph for the titration curve, because if you did, then you can figure it out if it is strA/strB or wkA/srtB or wkB/strA those pretty much all the options

and yes [oh-] =10^-7 and [h+]=10^-7 is correct but this is not what the ? is asking (those are one of MCAT favorites - this is absolutly right, with the exception that they did not ask this🙂

if have more ?s bug me (i think i am ok with chem🙂, or so i hope🙂

good luck