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K a – The Acid Dissociation Constant

K a – The Acid Dissociation Constant. Strong Acids and Bases. Recall: strong acids and bases completely dissociate (break apart into ions) when dissolved in water Example: HCl (aq) + NaOH (aq)  HOH (l) + NaCl (aq) is really H + + Cl - + Na + + OH -  H + + OH - + Na + + Cl -

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K a – The Acid Dissociation Constant

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  1. Ka – The Acid Dissociation Constant

  2. Strong Acids and Bases • Recall: strong acids and bases completely dissociate (break apart into ions) when dissolved in water Example: HCl (aq) + NaOH (aq) HOH (l) + NaCl(aq) is really H+ + Cl- + Na+ + OH-  H+ + OH- + Na+ + Cl- Thus, it is easy to calculate the pH or pOH of these solutions.

  3. Weak Acids and Bases • Ka: the acid dissociation constant • a measure of how much an acid breaks apart into its ions • it is similar to the equilibrium constant Keq

  4. Example #1 CH3COOH + H2O  H3O+ + CH3COO- Ka= [H3O+][CH3COO-] [CH3COOH] This Ka value now tells us the tendency of CH3COOH to ionize in water.

  5. Ka Values • Large Ka value = stronger acid (more ionization) • Example: Ka of HF = 6.8 x 10-4 • Small Ka value = weaker acid (less ionization) • Example: Ka of HCN = 6.2 x 10-10

  6. What do we use Ka values for? Since weak acids only partially dissociate, the calculation of pH using –log [H+] is not accurate. • Thus, Ka is used to determine the pH of weak acids. • You can also use the pH of weak acids to calculate Ka.

  7. Determining Ka – Example #1 Determine the Ka for a 0.10 M solution of HCOOH if [H3O+] = 4.2 x 10-3 M.

  8. Determining Ka – Whiteboards • Find the Ka of a 2.00 M HClO2 solution if [H3O+] = 0.14 M. • In a 0.50 M solution of a weak acid HX, the [H3O+] = 8.0 x 10-2 M. Calculate the acid dissociation constant. • Which acid is the stronger acid?

  9. Calculating Ka from pH – Example #2 A 0.10 M solution of lactic acid (CH3CHOHCOOH) has a pH of 2.44. Calculate the Ka of lactic acid.

  10. Calculating Ka from pH – Whiteboards • A student prepared a 0.0075 M solution of HNO2 and measured the pH to be 5.48. Calculate the Ka for HNO2. • A 0.020 M solution of niacin (C5H4NCOOH) has a pH of 3.26. What is the acid dissociation constant for niacin? • Which acid is the weaker acid?

  11. Calculating pH from Ka – Example #3 Determine the pH of a 0.30 M solution of CH3COOH, given Ka = 1.8 x 10-5.

  12. Calculating pH from Ka – Example #3 Ka = [H+][CH3COO-] = (x)(x) = 1.8 x 10-5 [CH3COOH] 0.30-x The x is so small in comparison to the Molarity of CH3COOH to begin with, we assume Molarity – x is negligible and just drop the x from the formula. (x)(x) = 1.8 x 10-5 0.30

  13. Calculating pH from Ka – Whiteboards • Calculate the pH of a 2.92 M HCN solution if the Ka = 6.2 x 10-10. • Determine the pH of a 0.095 M hypochlorous acid solution if the acid dissociation constant is 3.5 x 10-8. • Which acid is the weaker acid?

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