1 / 13

Understanding Acid Dissociation Constants in Chemistry

Learn about acid dissociation constants (Ka) for strong and weak acids, calculate pH using Ka values, and determine ionization tendencies. Practice solving problems with Ka and pH calculations in chemistry.

mariephillips
Download Presentation

Understanding Acid Dissociation Constants in Chemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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?

More Related