pH salts

Core Concept

A salt is an ionic compound formed when an acid reacts with a base, typically consisting of a cation (positive ion) from the base and an anion (negative ion) from the acid.

General Reaction: Acid + Base → Salt + Water

Key Tips

Salts can be acidic, basic, or neutral depending on their parent acid and base.
Hydrolysis of cations or anions determines the pH of the solution.
Use the strengths of the acid and base to predict the behavior of the salt in water.

Test Yourself

Assorted Multiple Choice

When solid ammonium chloride ($NH_4Cl$) is dissolved in pure water at $25^\circ\text{C}$, the resulting solution is found to be acidic ($\text{pH} < 7$). Which of the following ions is responsible for this behavior by undergoing a hydrolysis reaction with water?

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pH of Salts

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Qualitative Prediction of Salt pH

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Anion Hydrolysis (Basic Salts)

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Salts with Two Hydrolyzing Ions

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Quantitative pH Calculations for Salts

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pH of Salt Solutions

The pH of a salt solution depends on the strength of the parent acid and base:

  
    

Parent Acid
Parent Base
Solution Nature
pH Level
Example


Strong
Strong
Neutral
pH = 7
NaCl (from HCl + NaOH)

Strong
Weak
Acidic
pH < 7
NH4Cl (from HCl + NH3)

Weak
Strong
Basic
pH > 7
CH3COONa (from CH3COOH + NaOH)

Weak
Weak
Variable
Depends on Ka vs Kb
NH4CN (from HCN + NH3)


  

Determining pH for Weak Acid + Weak Base:

If $K_a > K_b$: The solution is acidic.
If $K_b > K_a$: The solution is basic.
If $K_a \approx K_b$: The solution is neutral.

Hydrolysis of Salts

Definition: Hydrolysis occurs when the anion or cation of a salt reacts with water to produce $H^+$ or $OH^-$, thereby affecting the solution's pH.

Key Cases:

Cation Hydrolysis
Cations derived from weak bases (e.g., $NH_4^+$, $Al^{3+}$) react with water to produce $H_3O^+$, making the solution acidic.
$$NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+$$
Anion Hydrolysis
Anions derived from weak acids (e.g., $CH_3COO^-$, $CO_3^{2-}$) react with water to produce $OH^-$, making the solution basic.
$$CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^-$$

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