Acids, Bases, pH and Buffers
Bronsted acids donate protons and bases accept them; pH = -log[H+], and a buffer resists pH change through a weak acid and its conjugate base.
What is Acids, Bases, pH and Buffers?
Bronsted acids donate protons and bases accept them; pH = -log[H+], and a buffer resists pH change through a weak acid and its conjugate base.
Key formula / rule: pH + pOH = 14 at 298 K.
Key points
- Compute the pH of strong and weak acid solutions.
- Apply the Henderson-Hasselbalch equation to buffer problems.
Common exam trap
Computing weak acid pH as if the acid were fully ionised.
Definitions
- Term
Acids, Bases, pH and Buffers
- Meaning
Bronsted acids donate protons and bases accept them; pH = -log[H+], and a buffer resists pH change through a weak acid and its conjugate base.
- Term
Acids, Bases, pH and Buffers — explanation
- Meaning
Weak electrolytes only partially ionise, so their pH follows from the dissociation constant rather than the formal concentration. The Henderson-Hasselbalch relation gives the buffer pH directly.
Learning objectives
Compute the pH of strong and weak acid solutions.
Apply the Henderson-Hasselbalch equation to buffer problems.
Formulae
- Key point
pH + pOH = 14 at 298 K.
- Key point
For a buffer, pH = pKa + log([salt]/[acid]).
- Key point
Strong acids ionise completely while weak acids do not.
Prerequisites
CHE-U1-EQ-T1-S1-C1
Common mistakes
Computing weak acid pH as if the acid were fully ionised.
Applying the buffer equation to a solution containing no conjugate pair.
Keywords
Acids
Bases
Buffers
Practice preview
Identify the conjugate base of H2PO4- in the following reaction: H2PO4-(aq) + H2O(l) <=> HPO4^2-(aq) + H3O+(aq)…
medium
A buffer solution contains 0.2 M CH3COOH (Ka = 1.8 x 10^-5) and 0.2 M CH3COONa. What is the pH of this buffer?…
medium
What is the pH of a solution formed by mixing 50 mL of 0.1 M NaOH with 50 mL of 0.2 M CH3COOH (Ka = 1.8 x 10^-5)?…
hard