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Chemistry: Atoms First 2e · Acid-Base Equilibria

Hydrolysis of Salts

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On this page 8 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Check yourself
  7. Study tools
  8. Sources & references

In 30 seconds

A salt such as sodium chloride dissolves in water to give a neutral solution, but many salts do not. When a salt contains an ion that is the conjugate of a weak acid or base, that ion reacts with water — a process called hydrolysis — and the solution turns acidic or basic. The general rule: an anion that is the conjugate base of a weak acid makes the solution basic; a cation that is the conjugate acid of a weak base makes it acidic; ions that are conjugates of strong acids or bases are spectators and leave pH alone.

Because the reacting ion's strength is set by the Ka × Kb = Kw relationship, the same calculation skills from the previous topic apply directly. This topic is where the acid-base framework becomes predictive: given only a salt's formula, you can classify its solution as acidic, basic, or neutral, and quantify the pH with a Ka or Kb calculation.

Why this matters

Predicting the pH of salt solutions is a practical skill in many settings:

  • Medicine and nursing: Intravenous saline is neutral, but many drug salts are chosen because of their hydrolysis behavior — for example, sodium bicarbonate is given intravenously to raise blood pH in metabolic acidosis. Understanding which ion reacts explains why.
  • Biology and food: Sodium dihydrogen phosphate and disodium hydrogen phosphate are the components of the phosphate buffer that holds intracellular pH steady. Baking soda (sodium bicarbonate) makes batters rise partly by shifting pH; pickling relies on acetic acid and its salts.
  • Aquariums, pools, and agriculture: The salts dissolved in water determine its pH and hardness; fish and plants are sensitive to both. Adding the wrong salt can shift pH enough to harm organisms.
  • Exams: Salt-hydrolysis classification ("acidic, basic, or neutral?") is one of the most frequently tested acid-base skills, and the pH calculation is a standard equilibrium problem.

The college version

Core Concepts

What "hydrolysis" means here

Hydrolysis literally means "splitting with water." In acid-base chemistry it refers to a dissolved ion reacting with water to produce H3O+ or OH−:

  • An ion acting as a weak acid toward water (e.g., NH4+): NH4+(aq) + H2O(l) ⇌ NH3(aq) + H3O+(aq), making the solution acidic.
  • An ion acting as a weak base toward water (e.g., CH3COO−): CH3COO-(aq) + H2O(l) ⇌ CH3COOH(aq) + OH-(aq), making the solution basic.

The key insight: an ion reacts with water only if it is the conjugate of a weak acid or base. Conjugates of strong acids (Cl−, NO3−, Br−) and strong bases (Na+, K+) are such feeble proton donors/acceptors that they do not hydrolyze; they are spectators.

The four cases of salt classification

  1. Salt of a strong acid + strong base (e.g., NaCl, KNO3): neither ion hydrolyzes → neutral, pH ≈ 7.
  2. Salt of a weak acid + strong base (e.g., NaCH3COO, NaCN, NaF): the anion is a conjugate base of a weak acid and hydrolyzes → basic, pH > 7.
  3. Salt of a strong acid + weak base (e.g., NH4Cl, NH4NO3): the cation is a conjugate acid of a weak base and hydrolyzes → acidic, pH < 7.
  4. Salt of a weak acid + weak base (e.g., NH4CH3COO): both ions hydrolyze; the pH depends on the relative strengths of Ka and Kb. For ammonium acetate, Ka(NH4+) = Kb(CH3COO−) = 5.6 × 10-10, so the solution is nearly neutral; unequal values tip the balance one way or the other.

Finding Ka or Kb for the reacting ion

The strength of the hydrolyzing ion is never a new constant — it comes from its conjugate's constant:

Kb(A−) = KwKa(HA)   and   Ka(BH+) = KwKb(B)

For example, acetate ion is the conjugate base of acetic acid (Ka = 1.8 × 10-5), so Kb(CH3COO−) = 1.0 × 10-14/1.8 × 10-5 = 5.6 × 10-10. Cyanide, conjugate base of the much weaker HCN (Ka = 4.9 × 10-10), gets a much larger Kb = 2.0 × 10-5, which is why sodium cyanide solutions are strongly basic while sodium acetate solutions are only mildly basic.

Amphiprotic anions: a special case

Anions like HCO3− and H2PO4− can act as either an acid or a base in water — they are amphiprotic. Which behavior wins is decided by comparing two numbers: their ability to donate a proton (measured by their own Ka, the second ionization of the parent acid) against their ability to accept one (measured by Kb = Kw/Ka1 of the parent acid). For bicarbonate, Kb = 2.3 × 10-8 is larger than Ka = 5.6 × 10-11, so bicarbonate solutions (like baking soda) are mildly basic.

How It Works / Step-by-Step Process

Worked example 1: pH of a basic salt solution

Problem. Calculate the pH of 0.50 M sodium acetate, NaCH3COO. Acetic acid has Ka = 1.8 × 10-5; assume 25 °C.

Solution.

  1. Dissolve and classify: NaCH3COO → Na+ + CH3COO−. Sodium is a spectator; acetate hydrolyzes as a base:

CH3COO-(aq) + H2O(l) ⇌ CH3COOH(aq) + OH-(aq)

  1. Get the base constant from the conjugate relationship:

Kb = KwKa = 1.0 × 10-141.8 × 10-5 = 5.6 × 10-10

  1. Let x = [OH−] = [CH3COOH] at equilibrium; [CH3COO−] = 0.50 - x. Write and solve:

Kb = x20.50 - x ≈ x20.50 = 5.6 × 10-10   ⇒  x = (5.6 × 10-10)(0.50) = 1.7 × 10-5 M

Check: 1.7 × 10-5/0.50 = 0.0034% < 5%, so the approximation is fine.

  1. Convert: pOH = -log(1.7 × 10-5) = 4.78, so

pH = 14.00 - 4.78 = 9.22

The 0.50 M solution of a salt of a weak acid and strong base is clearly basic.

Worked example 2: pH of an acidic salt solution

Problem. Calculate the pH of 0.20 M ammonium chloride, NH4Cl. Ammonia has Kb = 1.8 × 10-5.

Solution.

  1. Dissolve and classify: NH4Cl → NH4+ + Cl−. Chloride is a spectator; ammonium hydrolyzes as an acid:

NH4+(aq) + H2O(l) ⇌ NH3(aq) + H3O+(aq)

  1. Derive the acid constant:

Ka(NH4+) = KwKb(NH3) = 1.0 × 10-141.8 × 10-5 = 5.6 × 10-10

  1. Let x = [H3O+] = [NH3]; [NH4+] = 0.20 - x. Write and solve:

Ka = x20.20 - x ≈ x20.20 = 5.6 × 10-10   ⇒  x = (5.6 × 10-10)(0.20) = 1.1 × 10-5 M

  1. Convert:

pH = -log(1.1 × 10-5) = 4.98

Ammonium chloride solutions are acidic — the mirror image of sodium acetate.

Worked example 3: classifying salts without calculation

Problem. Predict whether 0.10 M solutions of KNO3, NaCN, and NH4NO3 are acidic, basic, or neutral, and rank the two reactive ones by pH.

Solution.

  1. KNO3: K+ and NO3− are both conjugates of strong base/strong acid → spectators → neutral, pH ≈ 7.
  2. NaCN: sodium is a spectator; cyanide is the conjugate base of weak HCN → hydrolyzes → basic. Its Kb = Kw/Ka = 1.0 × 10-14/4.9 × 10-10 = 2.0 × 10-5.
  3. NH4NO3: nitrate is a spectator; ammonium is the conjugate acid of weak ammonia → hydrolyzes → acidic, with Ka = 5.6 × 10-10.
  4. Ranking: because Kb(CN−) = 2.0 × 10-5 is far larger than Ka(NH4+) = 5.6 × 10-10, a 0.10 M cyanide solution is more strongly basic (pH well above 11) than the ammonium solution is acidic (pH near 5). The magnitudes of the constants, not the salt's formula alone, set how far the pH moves.

Common Confusions

Do Not ConfuseWithDifference
"Salts are neutral"Reality for most saltsOnly salts of strong acid + strong base (e.g., NaCl) are neutral; most salts hydrolyze.
NH4Cl being basicIts ammonia originAmmonium is an acid, not a base; NH4Cl solutions are acidic.
NaCH3COO being acidicIts acetic-acid originAcetate is the base here; sodium acetate solutions are basic.
Both ions hydrolyzing (weak + weak)One ion controlling pHBoth react, but the larger of Ka vs Kb (of the two ions) decides the pH direction.
Using Ka of the saltDeriving Kb from the parent acidThe hydrolyzing ion's constant must be derived via Kw; the parent acid's Ka alone is not the ion's constant.
DissolvingHydrolysisDissolving just separates ions; hydrolysis is the subsequent reaction of those ions with water.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

When a salt dissolves, it splits into two ions, like a dance couple separating. Most of the time the ions just swim around and the water stays neutral. But if one partner is a "proton grabber" or a "proton giver," it starts trading hydrogens with the water and changes how acidic the water is. A grabber makes the water more soapy (basic); a giver makes it more sour (acidic). Ions that come from strong acids or strong bases are too shy to trade at all — they just float along.

Key takeaways

  • Hydrolysis: a dissolved ion reacts with water to produce H3O+ or OH−; only conjugates of weak acids/bases hydrolyze.
  • Classification: strong + strong → neutral; weak acid + strong base → basic; strong acid + weak base → acidic; weak + weak → depends on relative Ka vs Kb.
  • Spectator ions: Na+, K+, Cl−, NO3−, Br− do not affect pH.
  • Kb(A−) = Kw / Ka(HA) and Ka(BH+) = Kw / Kb(B) — never look up the ion's constant, derive it.
  • The weaker the parent acid, the stronger the hydrolyzing anion (e.g., CN− > CH3COO− in basicity).
  • Amphiprotic anions: compare the anion's own Ka with its Kb = Kw/Ka1 to decide acidic vs basic.
  • To calculate pH: set up the hydrolysis equilibrium, write Ka or Kb, approximate when ionization < 5%, then convert to pH.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

  1. Why is a solution of NaCl neutral while NaCN is basic? Identify the reacting ion in each.

    Show answer

    NaCl's ions (Na+, Cl−) are conjugates of a strong base and strong acid, so neither hydrolyzes. In NaCN, cyanide is the conjugate base of weak HCN and hydrolyzes to produce OH−.

  2. Write the hydrolysis reaction that makes NH4Cl solutions acidic.

    Show answer

    NH4+(aq) + H2O(l) ⇌ NH3(aq) + H3O+(aq).

  3. Sodium fluoride (Ka of HF ≈ 6.3 × 10-4) — acidic, basic, or neutral? Why?

    Show answer

    Basic: fluoride is the conjugate base of a weak acid (HF) and hydrolyzes to produce OH−; sodium is a spectator.

  4. Derive Kb for fluoride ion from the data in question 3.

    Show answer

    Kb(F−) = Kw/Ka(HF) = 1.0 × 10-14/6.3 × 10-4 = 1.6 × 10-11.

  5. A salt contains HCO3−. How do you decide whether its solution is acidic or basic?

    Show answer

    Compare the anion's ability to act as an acid (its own Ka, the parent's second ionization) with its ability to act as a base (Kb = Kw/Ka1 of the parent). If Kb > Ka, basic; if Ka > Kb, acidic.

Keep learning

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

hydrolysis (of salts)
Reaction of a dissolved salt ion with water that changes pH
spectator ion
An ion that does not react with water
salt of weak acid + strong base
Salt whose anion is a conjugate base of a weak acid
salt of strong acid + weak base
Salt whose cation is a conjugate acid of a weak base
amphiprotic anion
Anion that can donate or accept a proton (HCO3−, H2PO4−)
Kₐ × Kb = Kw
Master relationship linking conjugate strengths

Sources & references

  1. openstax.org — Chemistry Atoms First 2e

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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