Salt hydrolysis – the pH of salt solutions
ChemistryAcids, Bases & TitrationAges 16–17Virtual lab
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Dissolve NaCl, KNO₃, NH₄Cl, AlCl₃, FeCl₃, CH₃COONa, Na₂CO₃, CH₃COONH₄ or NH₄CN in water at a chosen concentration, read a pH meter and an indicator, and see at particle level which ion reacts with water (NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺; CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻). The pH is solved exactly from the charge balance, with Kh = Kw/Ka or Kw/Kb at 25 °C. A results table sorts salts of strong or weak acids and bases into acidic, neutral and basic, and adding the conjugate acid or base shows the common-ion effect.
Lesson: Hydrolysis of ions in salt solutions and the pH of salt solutions
What it shows
When a salt dissolves, an ion that comes from a weak acid or a weak base reacts with water. NH₄⁺ gives a proton to water and forms H₃O⁺, so ammonium chloride is acidic; CH₃COO⁻ takes a proton from water and forms OH⁻, so sodium ethanoate is basic. Ions of strong acids and strong bases, such as Na⁺ and Cl⁻, do not react. The hydrolysis constant is Kh = Kw/Ka or Kw/Kb. The simulation solves the pH exactly at 25 °C for ideal dilute solutions; for Al³⁺ and Fe³⁺ only the first step is included.
How to use
Choose a Salt and its Concentration c, then press Dissolve and watch the pH meter, the colour of the Indicator and the magnifier, where a proton moves between an ion and a water molecule. Press Record to add the salt to the table, and compare acidic, neutral and basic salts at the same concentration. For NH₄Cl, CH₃COONa and Na₂CO₃, use the Added slider to see the common-ion effect. Answer the prediction and the pH calculation below the table.
Parameters you can change
- Salt sodium chloride, NaCl, potassium nitrate, KNO₃, ammonium chloride, NH₄Cl, aluminium chloride, AlCl₃, iron(III) chloride, FeCl₃, sodium ethanoate, CH₃COONa, sodium carbonate, Na₂CO₃, ammonium ethanoate, CH₃COONH₄, ammonium cyanide, NH₄CN
- Concentration c 0.001–1 mol/L
- Indicator Universal indicator, Litmus, Phenolphthalein, Methyl orange, Bromothymol blue
- Added conjugate acid or base 0–0.5 mol/L
Questions to explore
- Why is a solution of ammonium chloride acidic while sodium chloride is neutral?
- Why is ammonium ethanoate almost neutral although both of its ions react with water?
- How does the percentage of hydrolysed ions change when the salt solution is diluted?
