[fredodgers]

Hello everyone,

I have a conceptual question about the relationship between activity coefficients and titrations.  Suppose that we want to find the MW and pKa of some unknown weak acid.  We can standardize a strong base (find its molarity) and then titrate that weak acid and figure out the MW and pKa from the titration curve.  Let's say we do all of this without taking activity coefficients into account.  Then, the next time the titration is conducted, say activity coeffients are taken into account.  Would this make the value of the molecular weight and pKa deduced from the second titration  higher or lower than the values seen in the first titration?
Any ideas?

I appreciate the input!

[Borek]

Determined WM depends on the reaction stoichiometry, determined pK_a depends on the measure pH.

Does the stoichiometry depend on the activity coefficients?

Does the measured pH depend on the activity coefficients?

[fredodgers]

Determined WM depends on the reaction stoichiometry, determined pK_a depends on the measure pH.

Does the stoichiometry depend on the activity coefficients?

Does the measured pH depend on the activity coefficients?

With this intuition, it would seem that determined molecular weight would not change, but the determined pKa would change.  Is this correct?

[Borek]

With this intuition, it would seem that determined molecular weight would not change, but the determined pKa would change.  Is this correct?

Yes.

That is - I can construct examples of titrations in which the result of MW determination will be different, depending on the activity coefficients. But it requires assuming badly designed method, one in which someone ignores possible problems with exact equivalence point detection.