Equivalence Points

[pnoybballin]

Alright you guys so I'm a little confused on equivalence points between Strong by Strong and Weak by Strong. So the equivalence point is defined as the point at which the moles of titrant is equal to the moles of anylate. I'm confused with the half equivalence within a Weak by Strong titration curve. During the half equivalence point of a weak by strong titration curve, A- = HA making the pH = pKa but then why is this defined as the half equivalence point when A- - HA??

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[matth87]

Alright you guys so I'm a little confused on equivalence points between Strong by Strong and Weak by Strong. So the equivalence point is defined as the point at which the moles of titrant is equal to the moles of anylate. I'm confused with the half equivalence within a Weak by Strong titration curve. During the half equivalence point of a weak by strong titration curve, A- = HA making the pH = pKa but then why is this defined as the half equivalence point when A- - HA??

Henderson Hasselback

pH = pKa + log[base/acid]

when base = acid
log 1 = 0
pH = pKa

or...

Ka = [A-][H+]/[HA]

rearrange
[H+]/[Ka] = [HA]/[A-]

When [HA]/[A-] = 1 we know [H+]/[Ka] = 1
[H+] = [Ka]
or
-log(H+) = -log(Ka)
pH = pKa

its the exact same formula just written different ways.

 

[pnoybballin]

Henderson Hasselback

pH = pKa + log[base/acid]

when base = acid
log 1 = 0
pH = pKa

I understand that but why is it at that point it is defined as the half equivalence point? Shouldnt it be the equivalence point since the moles of base = moles of acid?

 

[matth87]

I understand that but why is it at that point it is defined as the half equivalence point? Shouldnt it be the equivalence point since the moles of base = moles of acid?

Moles of base dose not = moles of acid at half equiv.

The amount of unprotenated to protenated forms of your acid's are equal.
[HA] and [A-] are just the protenated or unprotenated forms of your acid you started with.

The base you are adding is the NaOH you are titrating with. The equivalence point is defined when you have added as much [OH-] as you started with [H+] and the solution is neutralized.

 

[pnoybballin]

Moles of base dose not = moles of acid at half equiv.

The amount of unprotenated to protenated forms of your acid's are equal.
[HA] and [A-] are just the protenated or unprotenated forms of your acid you started with.

The base you are adding is the NaOH you are titrating with. The equivalence point is defined when you have added as much [OH-] as you started with [H+] and the solution is neutralized.

Oh man that clarifies a lot. Thanks a bunch!

 

[thebillsfan]

Moles of base dose not = moles of acid at half equiv.

The amount of unprotenated to protenated forms of your acid's are equal.
[HA] and [A-] are just the protenated or unprotenated forms of your acid you started with.

The base you are adding is the NaOH you are titrating with. The equivalence point is defined when you have added as much [OH-] as you started with [H+] and the solution is neutralized.

When you say moles of base=moles of acid at equivalence pt, you really mean EQUIVALENTs of base = equiv of acid, correct?

 

[lDanny]

had a quick question about this...

so how are you suppose to determine the eq point? Is it just the average of pka of the acid and the pH of the titrant base? how do you pick the indicator for an unknown? got a little confused on it again lol. thanks