A-Level Chemistry Revision Notes
Acid-Base Chemistry: pH, Buffers, and Titrations
Harshitha Gowda
26-06-2025
Brønsted–Lowry Acid-Base Theory
• Acid: A substance that donates a proton (H+ ).
• Base: A substance that accepts a proton (H+ ).
This theory focuses on the transfer of a proton between reactants and applies to reactions
in aqueous and non-aqueous media.
General Reaction Form
HA + B ⇌ A− + HB+
• HA: Acid (donates H+ )
• B: Base (accepts H+ )
• A– : Conjugate base of HA
• HB+ : Conjugate acid of B
Example Reaction
−
NH3 + H2 O ⇌ NH+
4 + OH
• NH3 : Base (accepts H+ )
• H2 O: Acid (donates H+ )
• NH4 + : Conjugate acid
• OH– : Conjugate base
• Conjugate Acid: The species formed after a base accepts a proton.
• Conjugated Base: The species formed after an acid donates a proton.
• Every acid-base reaction involves two conjugate acid-base pairs.
• A substance that can act as both an acid and a base is called amphoteric (e.g.,
water).
1
, What is pH?
pH is a measure of the concentration of hydrogen ions [H + ] in a solution. It indicates
how acidic or basic a solution is.
pH = − log[H + ]
• pH ¡ 7: Acidic solution
• pH = 7: Neutral solution
• pH ¿ 7: Basic solution
Example: If [H + ] = 1 × 10−3 mol/L, then
pH = − log(10−3 ) = 3
What is Ka?
The acid dissociation constant Ka quantifies the extent to which a weak acid dissociates
in solution.
[H + ][A− ]
Ka =
[HA]
Where:
• [HA] = concentration of undissociated acid
• [H + ] = concentration of hydrogen ions
• [A− ] = concentration of conjugate base
Interpretation:
• High Ka → more dissociation → stronger weak acid
• Low Ka → less dissociation → weaker acid
What is pKa?
To make comparison easier, we convert Ka into pKa:
pKa = − log Ka
Interpretation:
2
Acid-Base Chemistry: pH, Buffers, and Titrations
Harshitha Gowda
26-06-2025
Brønsted–Lowry Acid-Base Theory
• Acid: A substance that donates a proton (H+ ).
• Base: A substance that accepts a proton (H+ ).
This theory focuses on the transfer of a proton between reactants and applies to reactions
in aqueous and non-aqueous media.
General Reaction Form
HA + B ⇌ A− + HB+
• HA: Acid (donates H+ )
• B: Base (accepts H+ )
• A– : Conjugate base of HA
• HB+ : Conjugate acid of B
Example Reaction
−
NH3 + H2 O ⇌ NH+
4 + OH
• NH3 : Base (accepts H+ )
• H2 O: Acid (donates H+ )
• NH4 + : Conjugate acid
• OH– : Conjugate base
• Conjugate Acid: The species formed after a base accepts a proton.
• Conjugated Base: The species formed after an acid donates a proton.
• Every acid-base reaction involves two conjugate acid-base pairs.
• A substance that can act as both an acid and a base is called amphoteric (e.g.,
water).
1
, What is pH?
pH is a measure of the concentration of hydrogen ions [H + ] in a solution. It indicates
how acidic or basic a solution is.
pH = − log[H + ]
• pH ¡ 7: Acidic solution
• pH = 7: Neutral solution
• pH ¿ 7: Basic solution
Example: If [H + ] = 1 × 10−3 mol/L, then
pH = − log(10−3 ) = 3
What is Ka?
The acid dissociation constant Ka quantifies the extent to which a weak acid dissociates
in solution.
[H + ][A− ]
Ka =
[HA]
Where:
• [HA] = concentration of undissociated acid
• [H + ] = concentration of hydrogen ions
• [A− ] = concentration of conjugate base
Interpretation:
• High Ka → more dissociation → stronger weak acid
• Low Ka → less dissociation → weaker acid
What is pKa?
To make comparison easier, we convert Ka into pKa:
pKa = − log Ka
Interpretation:
2
