CHEM1010 W2 L2

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.