Chapter 15.3

Objectives

• Define strong acids and bases.

• List the species that are strong acids and bases.

• Calculate the concentrations of species, the pH, and the pOH of solutions of strong acids and bases.

Strong Acids

• Strong acids ionize completely (100%) in solution.

• General equation for the ionization of a strong acid:

  • $HA(aq) + H<em>2O(l) \rightarrow H</em>3O^+(aq) + A^-(aq)$

Common Strong Acids

1. HCl - Hydrochloric acid

2. HBr - Hydrobromic acid

3. HI - Hydroiodic acid

4. HNO₃ - Nitric acid

5. HClO₄ - Perchloric acid

6. H₂SO₄ - Sulfuric acid

   • For sulfuric acid:

     • $H<em>2SO</em>4(aq) + H<em>2O(l) \rightarrow H</em>3O^+(aq) + HSO_4^-(aq)$

• It is important to MEMORIZE these strong acids.

Calculating pH from Strong Acids

Example: pH Calculation for HCl

• For a 0.050 M HCl solution:

  • Initial concentration before ionization;

  • $[HCl]_{initial} = 0.050 ext{ mol/L}$

• The pH can be calculated using:

  • $pH = - \log[H_3O^+]$

  • So:

  • $pH = -\log(0.050) = 1.30$

• Strong acid solutions typically have a pH range of 0-2.

Strong Bases

• Strong bases also ionize completely (100%) in solution.

• Common strong bases include the group IA and IIA hydroxides and oxides.

• General equation for the ionization of a strong base:

  • $NaOH(s) \rightarrow Na^+(aq) + OH^-(aq)$

  • Another example:

  • $Li<em>2O(s) + H</em>2O(l) \rightarrow 2Li^+(aq) + 2OH^-(aq)$

• Common strong bases include:

  • LiOH

  • NaOH

  • KOH

  • RbOH

  • CsOH

  • Sr(OH)₂

  • Ba(OH)₂

Calculating pH for Strong Bases

Example: pH Calculation for Barium Hydroxide

• For a 0.035 M barium hydroxide solution:

  • $Ba(OH)_2(s) \rightarrow Ba^{2+}(aq) + 2OH^-(aq)$

• The concentration of hydroxide ions is:

  • $[OH^-] = 2 \times 0.035 ext{ M} = 0.070 ext{ M}$

• To find the pH, use:

  • $pOH = -\log(0.070) = 1.15$

• Since $pH + pOH = 14$:

  • $pH + 1.15 = 14.00$

  • $pH = 14.00 - 1.15 = 12.85$

• Strong base solutions typically have a pH range of 12-14.

Additional pH Calculations

Example: Concentration from pH of HCl

• For an HCl solution with a pH of 3.75:

  • Calculate the concentration using:

  • $pH = 3.75$

  • $[H_3O^+] = 10^{-pH} = 10^{-3.75} = 1.78 \times 10^{-4} ext{ M}$

Example: Concentration from pH of Sr(OH)₂

• For a solution having a pH of 11.60:

  • $pH + pOH = 18.00$

  • This means:

  • $pOH = 18.00 - 11.60 = 6.40$

  • To calculate hydroxide ion concentration:

  • $[OH^-] = 10^{-pOH} = 10^{-6.40} = 3.98 \times 10^{-7} ext{ M}$

• The concentration of Sr(OH)₂ can be inferred:

  • Since each mole of Sr(OH)₂ produces two moles of OH⁻:

  • $[Sr(OH)_2] = \frac{[OH^-]}{2} = \frac{3.98 \times 10^{-7}}{2} = 0.0020 ext{ mol/L}$