Chemistry Class on pH and Acidity

pH Definition and Calculation

• pH is defined as the negative logarithm (base 10) of hydrogen ion concentration.
• Formula to calculate pH from pOH: pH = 14 - pOH.
• Formula to calculate pOH from pH: pOH = 14 - pH.

Methyl Orange Indicator

• Effective pH range: 3-5.
• Color in acidic solution: Red.
• Color in basic solution: Yellow.

pH Scale Limitations

• The pH scale is valid only within the range of 0-14.
• It is temperature-dependent, meaning values can change with temperature variations.

Water Self-Ionization

• Kw represents the self-ionization constant of water, indicating the balance of H+ and OH- ions.

Identifying Weak Acids and Bases

• Weak acids and bases can be identified through their dissociation constants, Kb (bases) and Ka (acids).
• If a value of Kb or Ka is provided, the substance is likely weak.

Calculating Concentrations

• For acids: [H+] = √(Ka × Molarity of the acid).
• For bases: [OH-] = √(Kb × Molarity of the base).

Reaction Dynamics with Added Acids/Bases

• Addition of a base (e.g., KOH) favors the reverse reaction, depleting hydroxide ions (OH-), resulting in a color change to blue.
• Addition of an acid leads to the formation of water from H+ and OH- ions, driving the forward reaction to replenish OH- ions, producing a yellow color change.

Indicator Suitability for Titrations

• Strong acid vs. Strong base: Use Methyl Orange, Phenolphthalein, or Litmus.
• Strong acid vs. Weak base: Use Methyl Orange.
• Weak acid vs. Strong base: Use Phenolphthalein.
• Weak acid vs. Weak base: No effective indicators.

Phenolphthalein Properties

• Effective pH range: 8-10.
• Color in basic solution: Pink.
• Color in acidic solution: Colorless.

Litmus Properties

• Effective pH range: 5-7.
• Color in acidic solution: Red.
• Color in basic solution: Blue.