pH salts

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Topic Summary & Highlights
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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

Practice 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.

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pH of Salt Solutions

The pH of a salt solution depends on whether the salt is formed from a:

  1. Strong acid and strong base: Neutral solution (pH=7\text{pH} = 7pH=7).

    • Example: NaCl from HClHClHCl and NaOHNaOHNaOH.

  2. Strong acid and weak base: Acidic solution (pH<7\text{pH} < 7pH<7).

    • Example: NH4ClNH_4ClNH4​Cl from HClHClHCl and NH3NH_3NH3​.

  3. Weak acid and strong base: Basic solution (pH>7\text{pH} > 7pH>7).

    • Example: CH3COONaCH_3COONaCH3​COONa from CH3COOHCH_3COOHCH3​COOH and NaOHNaOHNaOH.

  4. Weak acid and weak base: Depends on the relative strengths (KaK_aKa​ and KbK_bKb​) of the acid and base.

Hydrolysis of Salts

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

Key Cases:

  1. Cation hydrolysis:

    • Cations from weak bases (NH4+NH_4^+NH4+​, Al3+Al^{3+}Al3+) produce H+H^+H+, making the solution acidic.

    NH4++H2O⇌NH3+H3O+NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+NH4+​+H2​O⇌NH3​+H3​O+

  2. Anion hydrolysis:

    • Anions from weak acids (CH3COO−CH_3COO^-CH3​COO−, CO32−CO_3^{2-}CO32−​) produce OH−OH^-OH−, making the solution basic.

    CH3COO−+H2O⇌CH3COOH+OH−CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^-CH3​COO−+H2​O⇌CH3​COOH+OH−

Determining pH of Salts

Steps:

  1. Identify the acid and base from which the salt was formed.

  2. Determine the strength of the acid and base:

    • Strong acid: HCl, HNO_3​, H_2SO_4​

    • Strong base: NaOH, KOH, Ca(OH)_2​

    • Weak acid: CH_3COOH, H_2CO_3

    • Weak base: NH_3​, C_5H_5N

  3. Analyze the effect of hydrolysis:

    • If the cation hydrolyzes, the solution is acidic.

    • If the anion hydrolyzes, the solution is basic.

    • If neither hydrolyzes (strong acid and strong base), the solution is neutral.

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