CHEM1010 W2 L1.2

Overview of pH Calculations for Strong Acids and Bases

  • Focus on calculating the pH of strong acids and bases using dissociation principles.

  • Strong acids and bases fully dissociate in solution.

Strong Acids

  • Definition: A strong acid completely dissociates in a solution.

  • Key equation: pH = -log[H+]

  • Example 1:

    • 2.5 M HCl solution

    • Each mole of HCl provides one mole of H+.

      • Calculation:

        • [H+] = 2.5 M

        • pH = -log(2.5) = -0.4

    • Interpretation: Negative pH values indicate a very acidic solution.

  • Example 2: Same concentration, different volume

    • The pH remains the same (i.e., still -0.4) regardless of volume.

  • Example 3: 0.1 M Nitric acid

    • This also dissociates completely yielding a pH of 2, demonstrating a still very acidic solution.

Strong Bases

  • Definition: Strong bases are typically soluble hydroxides that completely dissociate.

  • Common strong bases: Hydroxides of Group 1 and Group 2 metals (e.g., NaOH, Ca(OH)2).

  • Example 1:

    • 2.5 M NaOH solution

    • Dissociates to give 2.5 M of OH- ions.

      • Calculation:

        • pOH = -log[OH-] = -log(2.5) = -0.4

        • Convert to pH: pH = 14 - pOH = 14 - (-0.4) = 14.4 (strongly basic solution).

  • Example 2: Different volume but same concentration yields the same pH (14.4).

  • Example 3: 0.1 M Calcium hydroxide

    • Produces two moles of OH- per mole of Ca(OH)2.

    • [OH-] = 0.2 M.

      • Calculation:

        • pOH = -log(0.2)

        • Convert to pH: pH = 14 - pOH.

    • Resulting pH indicates it’s a less basic solution than NaOH.

Summary of Lecture

  • Strong acids and bases fully dissociate to yield H+ or OH- ions.

  • Students should be confident in calculating pH and pOH for strong acids and bases.

  • Understanding the role of the dissociation constants (K_w) is essential for calculations involving weak acids and bases, which will be discussed in the next lecture.