Weak Acids

Acidity and Behavior,

the Vinegar Example

Two forms of the acid exist in solution

AcHHAcFree Acid Conjugate Base

Acetic Acid Acetate Ion

Crosses membranes easily Does not cross membranes

Volatile Non-volatile

Smells Less Smell

Weak Acids Dissociate Poorly as Seen by Their Dissociation Constants, Ka

][01.0][

10

][

]][[

4

HAcAc

K

HAc

AcHK

a

a

In other words, only one in a hundred protons dissociates!

And Its -log10, pKa

pKa = -log10Ka

Ka for acetic acid is 1.76 x 10-5 M,

pKa = -log 1.76 x 10-5 = ?

pKa for formic acid is 3.75, Ka = 10-pKa

= 10-3.75 = ?

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

μL Base Added pH

0 2.5

100 4.0

200 4.5

300 4.5

400 5.0

500 5.0

600 5.5

700 6.0

800 8.0

900 11.0

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

0

2

4

6

8

10

12

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

mequivalents OH- Added

pH

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

][

]][[

HAc

AcHKa

][

][log10 HA

AcpKpH a

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?

][

][10

28.48.2log

log

][

][

][

][log

2

10

10

10

HA

Acr

r

pKpHr

HA

Acr

HA

AcpKpH

a

a

but Not the pH

pKa 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

aTotal KHAcH ][][

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? OrWhen does [HAc] = [Ac-]?

][][

1

10

0log

][

][

0][

][log

0

10

10

AcHAc

r

r

r

rHA

Ac

HA

AcpKpH a

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

0

2

4

6

8

10

12

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

mequivalents OH- Added

pH

pKa[HAc] = [Ac-]

Mostly HAc Present

Mostly Ac- Present

Behavior of Vinegar

0

2

4

6

8

10

12

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

mequivalents OH- Added

pH

pH of solution barely changes when base added

Strong smell, crosses membranes

Does not smell, can’t cross membranes