pH scale
Definition of pH, pOH, and calculations for pH of strong and weak acid/base solutions, including mixtures.
What is pH scale?
A measure of the acidity or basicity of an aqueous solution, defined as the negative logarithm (base 10) of the hydrogen ion concentration.
Key formula / rule: pH Definition
Key points
- Define pH and pOH.
- Calculate pH and pOH for strong acids and bases.
- Calculate pH for weak acids and bases using Ka/Kb.
- Understand and apply the relationship pH + pOH = 14.
Common exam trap
Confusing log and antilog calculations.
Definitions
- Term
pH
- Meaning
A measure of the acidity or basicity of an aqueous solution, defined as the negative logarithm (base 10) of the hydrogen ion concentration.
- Term
pOH
- Meaning
A measure of the basicity or alkalinity of an aqueous solution, defined as the negative logarithm (base 10) of the hydroxide ion concentration.
- Term
Acid Dissociation Constant (Ka)
- Meaning
The equilibrium constant for the dissociation of a weak acid into its conjugate base and a proton.
- Term
Base Dissociation Constant (Kb)
- Meaning
The equilibrium constant for the dissociation of a weak base into its conjugate acid and a hydroxide ion.
- Term
Buffer Solution
- Meaning
A solution that resists changes in pH when small amounts of an acid or a base are added to it.
- Term
Ion Product of Water (Kw)
- Meaning
The product of the molar concentrations of hydrogen ions and hydroxide ions in pure water at a given temperature.
Learning objectives
Define pH and pOH.
Calculate pH and pOH for strong acids and bases.
Calculate pH for weak acids and bases using Ka/Kb.
Understand and apply the relationship pH + pOH = 14.
Explain the concept of buffer solutions and use the Henderson-Hasselbalch equation.
Formulae
- Name
pH Definition
- Note
Measures acidity.
- Expression
pH = -log₁₀[H⁺]
- Name
pOH Definition
- Note
Measures alkalinity.
- Expression
pOH = -log₁₀[OH⁻]
- Name
Ion Product of Water
- Note
Relates H+ and OH- concentrations.
- Expression
Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ (at 25°C)
- Name
Relationship between pH and pOH
- Note
Derived from Kw.
- Expression
pH + pOH = 14 (at 25°C)
- Name
pH of Strong Monoprotic Acid
- Note
Assumes complete dissociation.
- Expression
[H⁺] = Molarity of acid
- Name
pOH of Strong Monohydroxy Base
- Note
Assumes complete dissociation.
- Expression
[OH⁻] = Molarity of base
- Name
pH of Weak Monoprotic Acid
- Note
Cₐ is molar concentration of acid. Approximation valid if dissociation is small.
- Expression
[H⁺] ≈ √(Kₐ × Cₐ)
- Name
pOH of Weak Monohydroxy Base
- Note
Cb is molar concentration of base. Approximation valid if dissociation is small.
- Expression
[OH⁻] ≈ √(Kb × Cb)
- Name
Henderson-Hasselbalch Equation
- Note
For buffer solutions containing a weak acid and its conjugate base.
- Expression
pH = pKₐ + log₁₀([Salt]/[Acid])
Prerequisites
Basic understanding of acids and bases.
Logarithms and their properties.
Molarity and concentration calculations.
Chemical equilibrium concepts (for weak acids/bases).
Common mistakes
Confusing log and antilog calculations.
Forgetting the negative sign in the pH definition.
Assuming complete dissociation for weak acids/bases.
Incorrectly applying the Henderson-Hasselbalch equation.
Not considering the ion product of water (Kw) in calculations.
Using the wrong concentration (e.g., mass instead of molarity).
Keywords
pH
pOH
Acidity
Alkalinity
Hydrogen ion concentration
Hydroxide ion concentration
Ion product of water
Ka
Kb
Buffer solution
Henderson-Hasselbalch equation
Strong acid
Weak acid
Strong base
Weak base
Practice preview
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