The pKa Scale

Acid Dissociation Constant, Ka

• Ka, is a measure of how readily H+ ions are released

HA(aq) + H2O (aq) H3O+

(aq) + A-(aq)

Ka = [H+][A-] [HA]

Ka Values

• Stronger acids – higher Ka values

• Weak acids – lower Ka values

Acid Ka

HCl 1.0 x 107

HNO3 4.0 x 101

HF 5.6 x 10-4

HCOOH 1.6 x 10-4

CH3COOH 1.7 x 10-5

pKa

• Since Ka values for weak acids are very small it is convenient to use the pKa scale where:

pKa = -log10 Ka

If Ka = 1.70 x 10-5 mol dm-3

Then pKa = -log10(1.70 x 10-5 )

pKa = 4.77

Ka Values & pKa Values

• Explain the trend in strength of these acids

• Calculate the pKa values of these acids

Acid Ka

CCl3COOH 0.2

HCOOH 1.5 x 10-4

CH3COOH 1.6 x 10-5

CH3CH2COOH 1.3 x 10-5

pKa Values

• Stronger acids – higher Ka values

• Stronger acids – lower pKa values

Acid Ka pKa

CCl3COOH 0.2 0.7

HCOOH 1.5 x 10-4 3.8

CH3COOH 1.6 x 10-5 4.8

CH3CH2COOH 1.3 x 10-5 4.9

• Calculate the pH of a 0.1M solution of a weak acid given that it has a pKa value of 4.76 at 298K.

pKa = -log10Ka

4.76 = -log10Ka

Ka = 1.738 x 10-5

[H+] = √ 1.738 x 10-5 x 0.1

Ka ≈ [H+]2 [HA]total

• Calculate the pH of a 0.1M solution of a weak acid given that it has a pKa value of 4.76 at 298K.

pH = -log10[H+]

pH = 2.88

[H+] = 1.318 x 10-3 mol dm-3

• A 0.01M solution of a weak acid, X, has a pH of 3.37 at 200K. Calculate the pKa value for X at this temperature.

pH = -log10[H+]

[H+] = 4.266 x 10-4

Ka ≈ [H+]2 [HA]total

Ka ≈ (4.266 x 10-4 )2 0.01

• A 0.01M solution of a weak acid, X, has a pH of 3.37 at 200K. Calculate the pKa value for X at this temperature.

pKa = -log10Ka

pKa = 4.74

Ka ≈ 1.8197 x 10-5