8.4 Calculations Involving Acidic Solutions

Strong Acids

• Take a bottle of 1.0 mol/L HCL(aq). It is a strong acid. What entities are in that bottle?

  • H+
  • Cl-
  • H2O (it auto-ionizes: can turn into hydronium or hydroxide, hydronium doesn’t matter that much with a strong acid)

• To calculate the pH:

  • In strong acids: we assume 100% ionization. In a solution of 0.25 mol/L of nitric acid, calculate the pH and pOH.

  

Weak Acids

• Find the percentage ionization (% of solute that ionizes when dissolved in solvent).
• % ionization = [H+] / [HA] x 100

Calculations Involving Acidic Solutions

• A solution of 0.10 mol/L methanoic acid has a pH of 2.38. What is the % ionization?

  

• Ethanoic acid has a % ionization of 1.3%. What is the Ka of a 0.1000 mol/L solution of ethnoic acid?

  

• Calculating pH of acidic solutions: (similar to equilibrium problems)

1. Identify major entities in the solution at equilibrium

2. Decide which of the major entities produces H+ ions

3. Identify major sources of H+ ions

4. Write the equilibrium constant equation for the reaction (primary source of H+)

5. Use ICE to determine changes at equilibrium, solve for x (don’t forget the 100 / 5% rule!)

6. Calculate the [H+]eq

7. Calculate pH

   1. Ex: What is the pH of 0.100 mol/L hypochlorous acid (See appendix B5 for Ka value)

   

• Calculating Ka of a weak acid:

1. Identify the source (s) of H+ ions

   1. Assume that although this is a weak acid, it’s still a much stronger acid than water

2. Write the ionization reaction and equilibrium constant equation

3. Construct your ICE table a. Initial acid concentration is given b. [H+ ]eq is determined by the pH

4. Calculate Ka

5. Check assumption (is Ka >> Kw?)

   1. Ex: A 0.050 mol/L solution of HC2H6NO2(aq) has a pH of 3.08. Calculate the Ka