Acids, Bases and pH
Arrhenius, Bronsted–Lowry and Lewis concepts, ionisation constants and pH scale. (Chemistry › Equilibrium, NEET UG syllabus.)
What is Acids, Bases and pH?
A substance that produces H⁺ ions (or H₃O⁺) when dissolved in water.
Key formula / rule: pH definition
Key points
- Define acids and bases according to Arrhenius, Brønsted-Lowry, and Lewis theories.
- Identify conjugate acid-base pairs.
- Calculate pH, pOH, [H⁺], and [OH⁻] for strong acids and bases.
- Calculate Ka, Kb, pKa, and pKb for weak acids and bases.
Common exam trap
Confusing Ka/Kb with pKa/pKb (Ka/Kb are dissociation constants, pKa/pKb are their negative logarithms).
Definitions
- Term
Acid (Arrhenius)
- Meaning
A substance that produces H⁺ ions (or H₃O⁺) when dissolved in water.
- Term
Base (Arrhenius)
- Meaning
A substance that produces OH⁻ ions when dissolved in water.
- Term
Acid (Brønsted-Lowry)
- Meaning
A species that donates a proton (H⁺).
- Term
Base (Brønsted-Lowry)
- Meaning
A species that accepts a proton (H⁺).
- Term
Conjugate Acid-Base Pair
- Meaning
Two species that differ from each other by the presence or absence of a proton (H⁺).
- Term
Acid (Lewis)
- Meaning
A species that accepts an electron pair.
- Term
Base (Lewis)
- Meaning
A species that donates an electron pair.
- Term
pH
- Meaning
A measure of the hydrogen ion concentration in a solution, defined as the negative logarithm (base 10) of [H⁺].
- Term
pOH
- Meaning
A measure of the hydroxide ion concentration in a solution, defined as the negative logarithm (base 10) of [OH⁻].
- Term
Acid Dissociation Constant (Ka)
- Meaning
The equilibrium constant for the dissociation of a weak acid in water.
- Term
Base Dissociation Constant (Kb)
- Meaning
The equilibrium constant for the dissociation of a weak base in water.
- Term
Ion Product of Water (Kw)
- Meaning
The product of the molar concentrations of H⁺ and OH⁻ ions in water at a specific temperature.
Learning objectives
Define acids and bases according to Arrhenius, Brønsted-Lowry, and Lewis theories.
Identify conjugate acid-base pairs.
Calculate pH, pOH, [H⁺], and [OH⁻] for strong acids and bases.
Calculate Ka, Kb, pKa, and pKb for weak acids and bases.
Relate Ka, Kb, Kw, pKa, pKb, pH, and pOH.
Predict the relative strengths of acids and bases based on their Ka/Kb or pKa/pKb values.
Formulae
- Name
pH definition
- Note
Measures hydrogen ion concentration
- Expression
pH = -log₁₀[H⁺]
- Name
pOH definition
- Note
Measures hydroxide ion concentration
- Expression
pOH = -log₁₀[OH⁻]
- Name
Ion product of water (at 25°C)
- Note
Kw varies with temperature
- Expression
Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴
- Name
pH and pOH relationship (at 25°C)
- Note
Derived from Kw
- Expression
pH + pOH = 14
- Name
Acid dissociation constant
- Note
For a weak acid HA ⇌ H⁺ + A⁻
- Expression
Ka = ([H⁺][A⁻]) / [HA]
- Name
Base dissociation constant
- Note
For a weak base BOH ⇌ B⁺ + OH⁻
- Expression
Kb = ([B⁺][OH⁻]) / [BOH]
- Name
pKa definition
- Note
Negative logarithm of Ka
- Expression
pKa = -log₁₀Ka
- Name
pKb definition
- Note
Negative logarithm of Kb
- Expression
pKb = -log₁₀Kb
- Name
Conjugate pair relationship (Ka, Kb)
- Note
For a conjugate acid-base pair at a given temperature
- Expression
Ka × Kb = Kw
- Name
Conjugate pair relationship (pKa, pKb)
- Note
For a conjugate acid-base pair at 25°C
- Expression
pKa + pKb = 14
Prerequisites
Basic understanding of chemical equilibrium.
Knowledge of logarithms and exponential functions.
Stoichiometry and concentration calculations (molarity).
Concept of dissociation and ionization.
Common mistakes
Confusing Ka/Kb with pKa/pKb (Ka/Kb are dissociation constants, pKa/pKb are their negative logarithms).
Incorrectly applying log rules, especially with negative signs.
Misidentifying conjugate acid-base pairs.
Assuming pH 7 is always neutral, without considering temperature effects on Kw.
Not distinguishing between strong and weak acids/bases when calculating pH.
Forgetting that [H⁺] and [OH⁻] from water autoionization must be considered for very dilute solutions of strong acids/bases.
Keywords
Acid
Base
pH
pOH
Arrhenius
Brønsted-Lowry
Lewis
Conjugate Acid-Base Pair
Ka
Kb
pKa
pKb
Kw
Equilibrium
Practice preview
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