1/2 equiv point and titration of an acid vs. base

[wacki]

So the 1/2 equivalence point always equals the pKa whether you are titrating an acid or a base right?

Lets say you are titrating unknown acid Y w/ NaOH. The 1/2 equiv point is pH of 4.
pKa = 4
pKb = 10

I get that. But what are those numbers describing? 4 is the pKa of acid Y? 10 is the pKb of conjugate base Y?


Now flip the titration.

Lets say you are titrating unknown base Z w/ HCL. The 1/2 equiv point is pH of 9.
pKa = 5 ?
pKb = 9 ?

5 is the pKa of the conjugate acid of the base Z?
9 is the pKb of base Z?

---

Members don't see this ad.

 

[PantherPride]

Your first scenario is correct. Your second scenario however is wrong: the equation used to show that pH = pKa, the Henderson-Hasselbach Equation, uses the pH to find pKa, or the pOH to find pKb. So what ever the pH is for the half-equiv point can be used to find the pKa of the conjugate acid to base Z, but it can't be used for the pKb, since you are using the pH rather than pOH. Thus the pKa of the conj acid is 9, while the pKb of the base is 5

In case you don't understand why pH=pKa for the half point, here is the HH equation:

At the half-equivalence point, HA = A (since the solution is half titrated), so the log of 1 = 0.

 

[wacki]

ok so pH tells you the pKa or the association constant of the acid being titrated or conjugate acid of the base being titrated. The word acid is what is consistent.

ok so pOH tells you the pKb or the association constant of the base being titrated or conjugate base of the acid being titrated. The word base is what is consistent.

I think I get it.

Thanks!