1. Unit 5: Acids & Bases Lesson 4
Chemistry 40S
Unit 5: Acids & Bases
Lesson 4

2. Learning Outcomes
C : Write the equilibrium expression (Ka or Kb) from a balanced chemical equation.
C : Use Ka or Kb to solve problems for pH, percent dissociation, and concentration.

3. Acid Dissociation Constant (Ka)
Strong acids completely dissociate  do not establish equilibrium
Weak acids do not completely dissociate  do establish equilibrium
E.g. HA + H2O ⇌ H3O+ + A–
Can write an equilibrium law for weak acids

4. Acid Dissociation Constant (Ka)
HA + H2O ⇌ H3O+ + A–
𝐾 𝑐 = 𝐻3𝑂+ [𝐴−] 𝐻𝐴 𝐾 𝑎 = 𝐻3𝑂+ [𝐴−] 𝐻𝐴
Just like with Keq the value of Ka indicates the state of equilibrium
The larger the Ka = the larger the [P]  stronger acid & more dissociation

5. Base Dissociation Constant
Base Dissociation Constant (Kb) reflects the strength of the base and its ability to dissociate
Everything works the same as with Ka…

6. Dissociation Constant Problems
Example 1: A 0.10 mol/L solution of acetic acid is only partly ionized. If at equilibrium, the hydronium ion concentration is 1.3×10–3 mol/L, what is the acid dissociation constant, Ka?

7. Dissociation Constant Problems
Example 2: HA is a weak acid with a Ka of 7.3×10–8. What is the concentration of all species (HA, H3O+ and A–) if the initial concentration of HA is 0.50 mol/L?

8. Percent Dissociation Several ways to describe acids and bases:
Concentration
Strong or weak
Degree of dissociation
Can also be described by percent dissociation:
𝑝𝑒𝑟𝑐𝑒𝑛𝑡 𝑑𝑖𝑠𝑠𝑜𝑐𝑖𝑎𝑡𝑖𝑜𝑛= 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑑𝑖𝑠𝑠𝑜𝑐𝑖𝑎𝑡𝑒𝑑 𝑠𝑝𝑒𝑐𝑖𝑒𝑠 𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑎𝑐𝑖𝑑 𝑜𝑟 𝑏𝑎𝑠𝑒 x 100%

9. Percent Dissociation 𝑝𝑒𝑟𝑐𝑒𝑛𝑡 𝑑𝑖𝑠𝑠𝑜𝑐𝑖𝑎𝑡𝑖𝑜𝑛= [𝐻3𝑂+] [𝐻𝐴] x 100%

10. Percent Dissociation Problems
Example 3: Calculate the percent dissociation of a mol/L solution of formic acid (HCH2O2) if the hydronium ion concentration is 4.21×10–4 mol/L.

11. Percent Dissociation Problems
Example 4: Calculate the Kb of the hydrogen phosphate ion (HPO42–) if a 0.25 mol/L solution of sodium hydrogen phosphate is dissociated is 0.080%.

12. pH, pOH, Ka & Kb Problems
Example 5: What is the pH of a 0.10 mol/L solution of the imaginary acid, HA, if its Ka is 4.0×10–9.

13. pH, pOH, Ka & Kb Problems
Example 6: The pH of a 0.20 mol/L solution of CO32– ions, is What is the Kb of carbonate ions?