AV

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.

    for strong acids

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