Equivalence Points

pnoybballin

Junior Member
10+ Year Member
5+ Year Member
Nov 29, 2005
96
0
0
  1. Pre-Medical
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??
 

matth87

Full Member
May 8, 2009
289
1
0
  1. Pre-Medical
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.
 

matth87

Full Member
May 8, 2009
289
1
0
  1. Pre-Medical
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.
 
About the Ads

pnoybballin

Junior Member
10+ Year Member
5+ Year Member
Nov 29, 2005
96
0
0
  1. Pre-Medical
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

Unseasoned Veteran
10+ Year Member
5+ Year Member
Dec 23, 2008
778
0
101
  1. Pre-Medical
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?
 
This thread is more than 12 years old.

Your message may be considered spam for the following reasons:

  1. Your new thread title is very short, and likely is unhelpful.
  2. Your reply is very short and likely does not add anything to the thread.
  3. Your reply is very long and likely does not add anything to the thread.
  4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
  5. Your message is mostly quotes or spoilers.
  6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
  7. This thread is locked.