Weak Acids Acidity and Behavior, the Vinegar Example

Two forms of the acid exist in solution Free AcidConjugate Base Acetic AcidAcetate Ion Crosses membranes easilyDoes not cross membranes VolatileNon-volatile SmellsLess Smell

Weak Acids Dissociate Poorly as Seen by Their Dissociation Constants, Ka In other words, only one in a hundred protons dissociates!

And Its -log 10, pK a pKa = -log 10 K a K a for acetic acid is 1.76 x M, pK a = -log 1.76 x = ? pKa for formic acid is 3.75, K a = 10 -pKa = = ?

Table 1. Titration of 5% Acetic Acid With 1 M NaOH μL Base AddedpH

Figure 1. Titration of 5% Acetic Acid With 1 M NaOH

Which Form Will Be Present? Free acid form occurs at low pH Conjugate base occurs at high pH At some point amount of acid equals amount of base

Henderson-Hasselbalch Equation

Predicts the Ratio of Acid and Base, What is the ratio of Ac - to HAc at pH 2.8? (See example on the right) Practice: What is the ratio of Ac - to HAc at pH 6.8?

but Not the pH pK a is the pH where there are equal amounts of free acid and conjugate base pH is dependent on the concentration of acid as well as its pKa pKa ≠ pH of a solution, pH is usually <pKa The above formula is derived from the equilibrium constant equation assuming [H + ] = [Ac - ], and [HAc] >> [Ac - ] The lower the pKa, the less valid the last assumption is.

What’s going on at pKa? Or When does [HAc] = [Ac - ]? When pH = pKa you can simplify the notation by representing the ratio with r, replace it in the equation, and solve its value. The ratio is 1 only when

Landmarks on the titration curve pKa [HAc] = [Ac - ] Mostly HAc Present Mostly Ac - Present

Behavior of Vinegar pH of solution barely changes when base added Strong smell, crosses membranes Does not smell, can’t cross membranes