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

  • 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
A constant current is passed through an electrolytic cell for 45.0 minutes, delivering a total charge of 8,100 Coulombs. How many moles of electrons were transferred during this process? (Faraday's constant = 96,485 C/mol e⁻)

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pH of 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:

  1. Cation Hydrolysis

    Cations derived from weak bases (e.g., $NH_4^+$, $Al^{3+}$) react with water to produce $H_3O^+$, making the solution acidic.

    $$NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+$$

  2. 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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