Equivalence Points - pH not always equal to 7?

[maxim3L]

Hi guys,

Quick question:
If the equivalence point is defined as the point at which equal amounts of acid and base are present in the solution, why isnt the pH 7 for both the titration of weak and strong acids with a strong base? My logic is that when equal amounts of H+ and OH- are present, pH must = 7 because it's neutral.

I'd appreciate your knowledge.

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[RPedigo]

Hi guys,

Quick question:
If the equivalence point is defined as the point at which equal amounts of acid and base are present in the solution, why isnt the pH 7 for both the titration of weak and strong acids with a strong base? My logic is that when equal amounts of H+ and OH- are present, pH must = 7 because it's neutral.

I'd appreciate your knowledge.

Let's say you have some strong base OH- titrating some weak acid HA.

You start out with 0mL of OH- added, so you just have HA:

HA
HA
HA
HA
HA

You start titrating with OH- and you get:

A-
HA
HA
HA
HA

Since the H+ from the weak acid and the OH- that is being introduced react to form water.

Keep titrating:

A-
A-
A-
HA
HA

Almost there.

A-
A-
A-
A-
A-

Now we have reached the equivalence point. You can see that there is a bunch of weak base left in the solution (A-), so the pH of a weak base solution is expected to be greater than 7. Hope this makes sense.

 

[maxim3L]

Thanks man, that was very insightful. It makes sense that you're left with alot of A- in H20 making the solution basic.

Another question:Wouldn't this apply to a strong acid titrated with a strong base as well though? For example, in the titration of HCl with NaOH, aren't you left with alot of Cl- in H2o?

 

[ballofnerves]

Thanks man, that was very insightful. It makes sense that you're left with alot of A- in H20 making the solution basic.

Another question:Wouldn't this apply to a strong acid titrated with a strong base as well though? For example, in the titration of HCl with NaOH, aren't you left with alot of Cl- in H2o?

Yeah but Cl- is such a weak, weak base that hardly any HCl will reform. If HCl does form again, it will quickly dissociate into H+ and Cl- since HCl is a very strong acid.

 

[maxim3L]

Well that puts this issue to rest LOL. Thanks man.

 

[wrathofgod64]

maxim, this also confused me too. The equivalence point isn't exactly when the moles of acid = the moles of base in solution because when you titrate a weak acid with a strong base, there's more conjugate base in solution at the equivalence point. Rather, the equivalence point is when the # of moles of base added equals # of initial moles of acid.

 

[Bu2009]

maxim, this also confused me too. The equivalence point isn't exactly when the moles of acid = the moles of base in solution because when you titrate a weak acid with a strong base, there's more conjugate base in solution at the equivalence point. Rather, the equivalence point is when the # of moles of base added equals # of initial moles of acid.

Well based from my analytical chem class, I learned that during an acid base titration, if you start with a Strong Acid as what ur titrating in beaker, and have a strong base in your buret, titrant. At eq pt your PH would be 7. When you have weak acid as titrated, and strong base as titrant, you would have a pH > 7 (basic ph) at eq. If you have a strong acid as titrated, and weak base as titrant you would get a acidic ph at eq (PH<7). I don't know if that helps.

 

[engineeredout]

pH of 7 doesn't always mean equal concentrations of H+ and OH- either. That only holds true at STP. Other conditions (temperature specifically) cause the pH to differ even when the H+ and OH- concentrations are the same (neutral solution).