Quick question about Titrations

[DJtiesto]

I was watching some Gold Standard video and the guy was doing a titration of HCl with NaOH and then went on to say that the equivalence point is where the ph=pKA. Umm...I thought the equivlance point would be where the moles of NaOH added is equal to the moles of HCl present.

I thought the ph=pKa at half-equivalence point? Or is this only for titrations with a weak acid or base?

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[loveoforganic]

The vid is wrong. pH = pKa is only for weak acids/bases though.

 

[DJtiesto]

Ok thanks. I was watching that and I was like holy **** what is this guy talking about

 

[boaz]

The vid is wrong. pH = pKa is only for weak acids/bases though.

 

[BerkReviewTeach]

The vid is wrong. pH = pKa is only for weak acids/bases though.

Just to add a little caveat, that holds at the half equivalence point, not at the equivalence point.

 

[Tokspor]

The vid is wrong. pH = pKa is only for weak acids/bases though.

You are saying that pH = pKa at the half-equivalence point for weak acids and bases, right? Why isn't this true for strong acids and bases?

 

[Isoprop]

You are saying that pH = pKa at the half-equivalence point for weak acids and bases, right? Why isn't this true for strong acids and bases?

Because strong acids and bases behave differently.

We derived that pH = pKa for a weak acid/base because of the H-H equation.

pH = pKa + log (A-/HA)
log (A-/HA) = 0 at half equiv point.

For a strong acid, H-H equation fails because ALL of the acid is dissociated in water. Same for a strong base.

For instance, for a strong acid, HCl. All of the H+ dissociates. There is virtually no HCl left. Then, [Cl-]/[HCl] = infinity. Also, what is the pKa of a strong acid? A really, really small number (pretty much zero).

The good news is that finding the pH at half equiv point of a strong acid base titration is EASY. At half equiv point, half of the H+ is left! So divide the # of moles of H+ left by the new volume, and you get the [H+]. Then take the negative log.