Acid–base chemistry is one of the most calculation‑heavy topics at A2 — and one where a clear method makes all the difference. Logarithms, weak‑acid approximations, buffers that resist pH change… it can feel like a lot. But it’s built from a few core equations applied carefully. Here’s the essential toolkit.
Starting point: what pH actually means
pH is just a way of expressing hydrogen ion concentration on a manageable scale:
$$text{pH} = -log_{10}[text{H}^+]$$
And going back the other way:
$$[text{H}^+] = 10^{-text{pH}}$$
Two things to internalise:
- Because it’s a log scale, a change of one pH unit means a tenfold change in [H⁺].
- The lower the pH, the higher the [H⁺] — the negative sign flips the direction.
Get comfortable moving between pH and [H⁺] on your calculator both ways; nearly every question needs it.
Strong acids: the easy case
A strong acid (like HCl) fully dissociates, so [H⁺] equals the acid concentration directly. Finding the pH is a single step:
Example: 0.1 mol dm⁻³ HCl. $$text{pH} = -log_{10}(0.1) = 1.0$$
That’s it. Strong acids are the gentle introduction.
Weak acids: enter Ka and pKa
Most acids at A2 are weak — they only partially dissociate. We describe that with the acid dissociation constant, Ka:
$$K_a = frac{[text{H}^+][text{A}^-]}{[text{HA}]}$$
A larger Ka means a stronger weak acid (more dissociation). Because Ka values span huge ranges, we often use pKa:
$$text{pKa} = -log_{10}K_a$$
The smaller the pKa, the stronger the acid. (Same logic as pH — the log makes big numbers manageable.)
Finding the pH of a weak acid
Use two standard approximations (which the exam expects):
- [H⁺] ≈ [A⁻] — the acid produces them in equal amounts.
- [HA] at equilibrium ≈ [HA] initial — so little dissociates that the loss is negligible.
This simplifies Ka to:
$$K_a approx frac{[text{H}^+]^2}{[text{HA}]}$$
Rearrange for [H⁺], then take −log for pH:
$$[text{H}^+] = sqrt{K_a times [text{HA}]}$$
Learn this route — it’s the backbone of weak‑acid pH questions.
Buffers: solutions that resist pH change
A buffer is a solution that resists changes in pH when small amounts of acid or base are added. It’s made from a weak acid and its conjugate base (e.g. ethanoic acid + sodium ethanoate).
How it works — the pair acts as a reservoir:
- Add acid (H⁺)? The conjugate base (A⁻) mops it up: A⁻ + H⁺ → HA.
- Add alkali (OH⁻)? The weak acid (HA) neutralises it: HA + OH⁻ → A⁻ + H₂O.
Either way, the ratio shifts only slightly, so the pH barely moves. Being able to explain this equilibrium shift is a common extended‑answer mark.
Calculating buffer pH
Rearrange the Ka expression for the buffer:
$$[text{H}^+] = K_a times frac{[text{HA}]}{[text{A}^-]}$$
Substitute the acid and salt concentrations, then take −log for pH. Buffer calculations are very doable once you’re fluent with Ka.
Titration curves: read the shape
A pH titration curve plots pH as you add one solution to another. Recognise the four combinations:
- Strong acid–strong base: long, near‑vertical jump around pH 7.
- Weak acid–strong base: vertical section is higher (above 7); a buffer region appears before the equivalence point.
- Strong acid–weak base: equivalence below 7.
- Weak acid–weak base: no sharp vertical section (hard to detect an endpoint).
Two exam skills: identify the equivalence point (centre of the vertical section) and choose a suitable indicator whose colour‑change range falls within that vertical jump.
Examiner’s tip
The most common errors here are calculator slips with logs and mixing up the weak‑acid and buffer equations. Keep them straight:
- Weak acid alone → [H⁺] = √(Ka × [HA])
- Buffer (acid + its salt) → [H⁺] = Ka × [HA]/[A⁻]
And always show the [H⁺] step before converting to pH — method marks live in that working. Round only at the very end.
The bottom line
A2 acid–base chemistry is a set of linked equations applied with care:
- pH = −log[H⁺], and back with [H⁺] = 10⁻ᵖᴴ.
- Strong acids dissociate fully; weak acids use Ka and the two approximations.
- Buffers resist pH change via a weak acid / conjugate base pair.
- Titration curves reveal equivalence points and the right indicator.
Practise a mix of strong‑acid, weak‑acid, and buffer calculations, and the whole topic settles into a reliable routine.
If the logs and approximations feel shaky, we can work through them live until the method is automatic — this is exactly the kind of topic where a clear walkthrough transforms your confidence.
👉 Book a free intro call and let’s master acids, bases and buffers.