Sodium hydroxide and hydrobromic acid. NaOH (aq) +HBr (aq) NaBr (aq) +H 2 O (l)
The pK a Scale. Acid Dissociation Constant, K a K a, is a measure of how readily H + ions are...
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Transcript of The pK a Scale. Acid Dissociation Constant, K a K a, is a measure of how readily H + ions are...
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