Hydrolysis of Salts Study Pack
Kibin's free study pack on Hydrolysis of Salts includes a 7-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.
Last updated May 28, 2026
Hydrolysis of Salts Study Guide
Unpack how dissolved salts produce acidic, basic, or neutral solutions by examining ion hydrolysis, the roles of conjugate acids and bases like acetate and ammonium, and how Ka, Kb, and Kw determine pH outcomes for every salt type.
Key Takeaways
- •A salt dissolved in water can produce an acidic, basic, or neutral solution depending on whether its parent acid and base are strong or weak.
- •Hydrolysis occurs when an ion from a dissolved salt reacts with water molecules, either donating or accepting a proton to shift the pH away from 7.
- •Salts derived from a strong acid and a strong base do not hydrolyze and produce neutral solutions because neither the cation nor the anion reacts appreciably with water.
- •The conjugate base of a weak acid (e.g., acetate, CH₃COO⁻) is itself a weak base that accepts protons from water, producing OH⁻ and raising the pH above 7.
- •The conjugate acid of a weak base (e.g., ammonium, NH₄⁺) is itself a weak acid that donates protons to water, producing H₃O⁺ and lowering the pH below 7.
- •For salts of weak acids and weak bases, the pH depends on the relative magnitudes of Ka and Kb; if Ka > Kb the solution is acidic, if Kb > Ka it is basic, and if Ka ≈ Kb it is approximately neutral.
- •The hydrolysis equilibrium constants Kb (for anion hydrolysis) and Ka (for cation hydrolysis) are calculated using the water autoionization constant: Kw = Ka × Kb.
What Happens When a Salt Dissolves in Water
When an ionic compound dissolves in water, it dissociates completely into its constituent cations and anions, and those ions may or may not interact chemically with the solvent. Whether the resulting solution is acidic, basic, or neutral depends entirely on how each ion behaves toward water molecules.
Dissociation vs. Hydrolysis
- •Dissociation is a physical separation of ions; hydrolysis is a chemical reaction between those ions and water that produces H₃O⁺ or OH⁻.
- •Not all ions hydrolyze — spectator ions from strong acids (Cl⁻, NO₃⁻, ClO₄⁻) and strong bases (Na⁺, K⁺, Ca²⁺) do not react measurably with water.
Parent Acid–Base Strength as the Determining Factor
- •The tendency of an ion to hydrolyze is directly tied to the strength of the acid or base from which it was derived.
- •A weak acid produces a conjugate base strong enough to accept a proton from water; a strong acid produces a conjugate base too weak to do so.
- •Similarly, a weak base produces a conjugate acid strong enough to donate a proton to water; a strong base produces a conjugate acid too weak to do so.
Neutral Solutions: Salts of Strong Acids and Strong Bases
Salts formed by the complete reaction of a strong acid with a strong base dissolve to give solutions with a pH of 7 at 25 °C because neither ion participates in hydrolysis.
Why No pH Shift Occurs
- •The anion (e.g., Cl⁻ from HCl) is the conjugate base of a strong acid, meaning it has an extremely low affinity for protons and does not remove H⁺ from water.
- •The cation (e.g., Na⁺ from NaOH) has no tendency to donate protons and does not affect water's proton equilibrium.
- •Because neither ion reacts with water, the concentrations of H₃O⁺ and OH⁻ remain equal at 1.0 × 10⁻⁷ M, and the solution stays neutral.
Common Examples
- •Sodium chloride (NaCl), potassium nitrate (KNO₃), and calcium chloride (CaCl₂) all produce neutral aqueous solutions.
Basic Solutions: Salts of Weak Acids and Strong Bases
When a salt contains the conjugate base of a weak acid, that anion undergoes hydrolysis by accepting a proton from water, which releases hydroxide ions and raises the solution pH above 7.
Anion Hydrolysis Mechanism
- •The anion acts as a Brønsted–Lowry base, accepting H⁺ from H₂O: A⁻ + H₂O ⇌ HA + OH⁻.
- •Producing OH⁻ shifts the equilibrium of water autoionization, increasing [OH⁻] and decreasing [H₃O⁺], so the solution becomes basic.
Equilibrium Constant for Anion Hydrolysis
- •The equilibrium constant for this reaction is expressed as Kb for the anion and equals Kw divided by Ka of the parent weak acid (Kb = Kw / Ka).
- •A weaker parent acid has a smaller Ka, which gives its conjugate base a larger Kb, meaning the anion hydrolyzes more extensively and produces a more basic solution.
Worked Concept: Sodium Acetate (CH₃COONa)
- •Acetate ion (CH₃COO⁻) is the conjugate base of acetic acid (Ka = 1.8 × 10⁻⁵), so its Kb = 1.0 × 10⁻¹⁴ / 1.8 × 10⁻⁵ ≈ 5.6 × 10⁻¹⁰.
- •A 0.10 M sodium acetate solution has a pH above 7, confirming the basic nature of anion hydrolysis.
Acidic Solutions: Salts of Strong Acids and Weak Bases
When a salt contains the conjugate acid of a weak base, that cation undergoes hydrolysis by donating a proton to water, which produces hydronium ions and lowers the solution pH below 7.
Cation Hydrolysis Mechanism
- •The cation acts as a Brønsted–Lowry acid, donating H⁺ to H₂O: BH⁺ + H₂O ⇌ B + H₃O⁺.
- •The release of H₃O⁺ increases acidity, driving the pH below 7.
Equilibrium Constant for Cation Hydrolysis
- •The equilibrium constant is expressed as Ka for the cation and equals Kw divided by Kb of the parent weak base (Ka = Kw / Kb).
- •A weaker parent base has a smaller Kb, which gives its conjugate acid a larger Ka, resulting in a more acidic solution.
Worked Concept: Ammonium Chloride (NH₄Cl)
- •Ammonium ion (NH₄⁺) is the conjugate acid of ammonia (Kb = 1.8 × 10⁻⁵), so its Ka = 1.0 × 10⁻¹⁴ / 1.8 × 10⁻⁵ ≈ 5.6 × 10⁻¹⁰.
- •A 0.10 M ammonium chloride solution has a pH below 7, consistent with cation hydrolysis producing excess H₃O⁺.
Hydrated Metal Cations as Acids
- •Small, highly charged metal ions such as Al³⁺, Fe³⁺, and Cr³⁺ form coordinate complexes with water molecules (e.g., [Al(H₂O)₆]³⁺) that act as weak acids.
- •The high positive charge density on these metals polarizes O–H bonds in the coordinated water, making proton donation to bulk water favorable and producing acidic solutions.
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Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.
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Question 1 of 25
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Which of the following salts produces a neutral solution when dissolved in water at 25 °C?
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Concept 1 of 5
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Ion Hydrolysis
Explain what ion hydrolysis is in your own words. How is it different from simple dissociation, and why does it cause the pH of a salt solution to shift away from 7?
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