Strong Acids, Strong Bases, and pH/pOH Calculations in GOB Chemistry

Strong Acids and Strong Bases

Definition and Properties

Strong acids and strong bases are substances that completely ionize in water, producing high concentrations of hydronium ions (H+) or hydroxide ions (OH-), respectively. This complete ionization allows for straightforward calculations of pH and pOH in aqueous solutions.

• Strong acids: Substances that donate protons (H+) completely in water.

• Strong bases: Substances that accept protons or release hydroxide ions (OH-) completely in water.

Key Point: For strong acids and bases, the concentration of the acid or base equals the concentration of H+ or OH- ions, respectively.

Examples of Strong Acids and Bases

• Strong Acids: HCl, HBr, HI, HNO3, H2SO4, HClO4

• Strong Bases: NaOH, KOH, LiOH, Ca(OH)2, Ba(OH)2

Ionization and Concentration Calculations

When strong acids or bases dissolve in water, they dissociate completely. For example:

• HCl → H+ + Cl-

• NaOH → Na+ + OH-

For polyprotic acids or bases (e.g., Ba(OH)2), the number of ions produced depends on the formula:

• Ba(OH)2 → Ba2+ + 2OH-

pH and pOH Calculations

Formulas

• pH: $\text{pH} = -\log[\text{H}^+]$

• pOH: $\text{pOH} = -\log[\text{OH}^-]$

• Relationship: $\text{pH} + \text{pOH} = 14$ (at 25°C)

Calculation Steps

1. Identify the concentration of the strong acid or base.

2. For bases like Ba(OH)2, multiply the concentration by the number of OH- ions produced per formula unit.

3. Apply the pH or pOH formula using the calculated ion concentration.

Example Calculation

Example: If the concentration of Ba(OH)2 solution is 0.398 M, calculate its pOH.

• Ba(OH)2 dissociates to produce 2 moles of OH- per mole.

• OH- concentration: $0.398\,\text{M} \times 2 = 0.796\,\text{M}$

• pOH calculation: $\text{pOH} = -\log(0.796)$

• pOH ≈ 0.10

Practice Problems

• Find the molar concentration of HBrO3 solution with a pH of 4.34: $\text{[H}^+] = 10^{-4.34} = 4.57 \times 10^{-5}\,\text{M}$

• Calculate the pH of 25 mL of 5.45 × 10-4 M LiOH solution: $\text{pOH} = -\log(5.45 \times 10^{-4}) = 3.26$ $\text{pH} = 14 - 3.26 = 10.74$

• HI is a strong acid (Ka = 3.2 × 109). Calculate [H+], [OH-], pH, and pOH of a 7.1 × 10-4 M HI solution: $\text{[H}^+] = 7.1 \times 10^{-4}\,\text{M}$ $\text{pH} = -\log(7.1 \times 10^{-4}) = 3.15$ $\text{pOH} = 14 - 3.15 = 10.85$ $\text{[OH}^-] = 10^{-10.85} = 1.41 \times 10^{-11}\,\text{M}$

Common Ions of Strong Bases

Ions of Strong Bases	Examples
OH-	NaOH, KOH, LiOH
NH2-	NaNH2
O2-	Na2O

Calculation of pH or pOH

• For strong bases, use the concentration of the base to determine [OH-].

• For bases that release more than one OH- per formula unit, multiply accordingly.

Summary Table: Strong Acid and Base Calculations

Type	Formula	Ion Produced	Calculation
Strong Acid	HCl	H+	[H+] = acid concentration
Strong Base	Ba(OH)2	OH-	[OH-] = base concentration × 2
Strong Base	NaOH	OH-	[OH-] = base concentration

Additional info: The notes infer standard GOB Chemistry content on strong acids/bases and pH/pOH calculations, including examples and formulas, to provide a complete study guide.