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The pK a Scale

The pK a Scale. Acid Dissociation Constant, K a. K a , is a measure of how readily H + ions are released. HA (aq) + H 2 O (aq) H 3 O + (aq) + A - (aq). K a = [H + ][A - ] [HA]. K a Values. Stronger acids – higher K a values Weak acids – lower K a values. pK a.

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The pK a Scale

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  1. The pKa Scale

  2. 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]

  3. Ka Values • Stronger acids – higher Ka values • Weak acids – lower Ka values

  4. 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

  5. Ka Values & pKa Values • Explain the trend in strength of these acids • Calculate the pKa values of these acids

  6. pKa Values • Stronger acids – higher Ka values • Stronger acids – lower pKa values

  7. 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 Ka≈ [H+]2 [HA]total [H+] = √ 1.738 x 10-5 x 0.1

  8. Calculate the pH of a 0.1M solution of a weak acid given that it has a pKa value of 4.76 at 298K. [H+] = 1.318 x 10-3 mol dm-3 pH = -log10[H+] pH = 2.88

  9. 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

  10. 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. Ka≈1.8197 x 10-5 pKa= -log10Ka pKa = 4.74

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