pH, pOH, Kw & Titrations

Autoionization of Water Reaction

H2O(l) + H2O(l) ⇌ H3O+(aq) + OH-(aq)

Kw (Ion Product Constant for Water)

[H3O+][OH-] = 1.0 x 10^-14 (at 25°C)

pH Definition

-log[H3O+] (or -log[H+])

pOH Definition

-log[OH-]

Relationship between pH and pOH

pH + pOH = 14.00 (at 25°C)

Calculating [H3O+] from pH

[H3O+] = 10^-pH

Calculating [OH-] from pOH

[OH-] = 10^-pOH

Acidic Solution (pH)

pH < 7 (at 25°C)

Neutral Solution (pH)

pH = 7 (at 25°C)

Basic (Alkaline) Solution (pH)

pH > 7 (at 25°C)

pH of Strong Acid Solution

pH = -log[Initial Acid Concentration] (for monoprotic strong acids)

pOH of Strong Base Solution

pOH = -log[Initial Base Concentration] (for Group 1 hydroxides)

Titration (Purpose)

To determine the concentration of an unknown acid or base using a solution of known concentration.

Equivalence Point (Titration)

The point at which moles of acid = moles of base (stoichiometrically).

MAVA = MBVB (Titration Shortcut)

Only works if the mole ratio between acid and base is 1:1 in the balanced equation.

Titration Calculation (General Moles Method)

Liters known → Moles known → (mole ratio) → Moles unknown → (Molarity or Molar Mass unknown)

Equivalence Point pH (Strong Acid + Strong Base)

Approximately 7

Equivalence Point pH (Weak Acid + Strong Base)

Greater than 7

Equivalence Point pH (Strong Acid + Weak Base)

Less than 7

Indicator (Titration)

A substance that changes color at or near the equivalence point.

Half-Equivalence Point (Weak Acid Titration)

Point where [weak acid] = [conjugate base]; pH = pKa.