Worked example 1
Predict whether solutions of KNO₃, NH₄Br and KCN are acidic, basic or neutral.
Try it first: Split each salt and trace each ion to its parent.
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Acids, Bases & Aqueous EquilibriaAcids and BasesContent level: Core 20 min
Salt Hydrolysis and Predicting Solution pH
What you'll be able to do: Predict whether a salt solution is acidic, basic or neutral and calculate its pH.
Introduction
Dissolving a salt in water can quietly produce an acidic or basic solution. Deciding which requires nothing new, only tracing each ion back to the acid or base it came from.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Split a salt into its ions and classify each one
- Predict whether a salt solution is acidic, basic or neutral
- Calculate the pH of a hydrolysing salt solution
- Resolve cases where both ions react by comparing Ka and Kb
Lesson
Judge the ions, not the salt
NaCl, NH₄Cl and NaCH₃COO are all salts, yet their solutions have pH 7.0, about 5.1 and about 8.9 at 0.10 M. The difference lies entirely in which parent acid and base each ion came from.
Classifying cations
Na⁺, K⁺, Ca²⁺ and Ba²⁺ come from strong bases and do not react. NH₄⁺ and protonated amines are conjugate acids of weak bases and are acidic. Small, highly charged metal ions such as Al³⁺ and Fe³⁺ polarize their bound water molecules enough to release a proton, making solutions of AlCl₃ noticeably acidic.
Al(H₂O)₆³⁺ + H₂O ⇌ Al(H₂O)₅(OH)²⁺ + H₃O⁺
Classifying anions
Cl⁻, Br⁻, I⁻, NO₃⁻ and ClO₄⁻ come from strong acids and are neutral. F⁻, CH₃COO⁻, CN⁻, NO₂⁻ and CO₃²⁻ come from weak acids and are basic. HSO₄⁻ is the odd one out: it is itself a weak acid.
Add an H⁺ to the anion. If the result is a strong acid, the anion is neutral.
The four combinations
Strong acid + strong base gives neutral (NaCl). Strong acid + weak base gives acidic (NH₄Cl). Weak acid + strong base gives basic (NaF). Weak acid + weak base depends on which constant is larger.
When both ions react
For NH₄CN, compare Ka(NH₄⁺) = 5.6 × 10⁻¹⁰ with Kb(CN⁻) = 2.0 × 10⁻⁵. The base is far stronger, so the solution is basic. For ammonium acetate the two are nearly equal, so the solution is close to neutral.
Compare Ka with Kb directly. Do not compare Ka of the cation with Ka of the parent acid of the anion.
Doing the calculation
Once you know which ion reacts, the problem is an ordinary weak acid or weak base calculation. The salt concentration is the initial concentration of the reacting ion, and the needed constant comes from Kw divided by the partner constant.
Key ideas
Rule
Neutral cations
Group 1 and heavy group 2 cations from strong bases.
Rule
Acidic cations
NH₄⁺, protonated amines, and small highly charged metal ions like Al³⁺ and Fe³⁺.
Rule
Basic anions
Conjugate bases of weak acids only.
Rule
Both react
Compare Ka of the cation with Kb of the anion; the larger one decides.
Rule
Setting up
The salt molarity is the starting concentration of the reacting ion.
Equation
Kb of an anion
Kb = Kw / Ka(parent acid)
- Ka = of the weak acid the anion came from
Equation
Ka of a cation
Ka = Kw / Kb(parent base)
- Kb = of the weak base the cation came from
Worked examples
Worked example 2
Calculate the pH of 0.20 M NH₄Cl. Kb(NH₃) = 1.8 × 10⁻⁵.
Try it first: Identify the reacting ion and find its constant.
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Worked example 3
Is a solution of NH₄F acidic or basic? Ka(HF) = 6.8 × 10⁻⁴, Kb(NH₃) = 1.8 × 10⁻⁵.
Try it first: Both ions react, so you need two constants on the same footing.
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Common mistakes
Assuming every salt solution is neutral.
Why it's wrong: Only salts of a strong acid and a strong base are neutral.
Check instead: Classify both ions separately.
Treating Na⁺ or K⁺ as acidic.
Why it's wrong: They are large with low charge density and do not polarize water.
Check instead: Only small highly charged metal ions are acidic.
Using the parent acid Ka directly for the anion.
Why it's wrong: The anion is a base, so it needs Kb = Kw / Ka.
Check instead: Convert before building the ICE table.
Forgetting that the salt concentration is the ion concentration.
Why it's wrong: For a 1:1 salt they are equal, but for K₂CO₃ the cation count differs.
Check instead: Use the formula stoichiometry.
Claiming any solution containing an ammonium ion is strongly acidic.
Why it's wrong: Ka is only 5.6 × 10⁻¹⁰, so the effect is mild.
Check instead: Calculate rather than assume.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Every salt separates into a cation and an anion, and each ion is judged separately. A cation is acidic if it is the conjugate acid of a weak base (such as NH₄⁺) or a small highly charged metal ion (such as Al³⁺ or Fe³⁺) whose hydrated form releases a proton; group 1 and heavier group 2 cations are neutral spectators. An anion is basic if it is the conjugate base of a weak acid, and neutral if it comes from a strong acid. Strong acid with strong base gives a neutral salt, strong acid with weak base gives an acidic salt, and weak acid with strong base gives a basic salt. When both ions react, compare Ka of the cation with Kb of the anion: whichever is larger wins. Quantitatively, look up Ka or Kb for the reacting ion, run the ordinary weak acid or weak base calculation, and remember the salt concentration is the starting concentration of that ion.
- Classify a salt by examining each ion separately
- Ions from strong acids and strong bases are spectators
- NH₄⁺ and small highly charged metal cations are acidic
- Conjugate bases of weak acids make solutions basic
- When both ions react, the larger of Ka and Kb decides
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.