Acidic and Basic Salts

Purpose

To help the class learn to identify ions as weak acids and bases.

Procedure

1. Solutions Preparation:

• Prepare 1 M solutions of:
  • Sodium chloride (NaCl)
  • Sodium acetate (Na+CH3COO-)
  • Sodium carbonate (Na2CO3)
  • Aluminum chloride (AlCl3)
  • Ammonium chloride (NH4Cl)

1. Prediction Activity:

• Display a universal indicator color chart on the document camera.
• Allow students to discuss predictions with neighbors.
• Have students raise hands to show their predictions.

1. Testing:

• Test the solutions by adding universal indicator.
• Discuss conjugate pairs based on the results.

1. Bonus Activity:

• Fill a large graduated cylinder with water and universal indicator.
• Repeat the prediction procedure using dry ice.
• Note that dilute coloration is most visible on the projector.
• Important: Test sodium acetate ahead of time, as it may need to be made fresh due to bacterial spoilage.

Ionization Constants

Ammonium Chloride (NH4Cl)

NH4⁺​ + H2​O ⇌ NH3 ​+ H3​O⁺

K_a​ = 7.5×10⁻³

pK_a​ = 2.12

Sodium Chloride (NaCl)

Neutral salt; does not affect pH.

Aluminum Chloride (AlCl3)

Al³⁺ + H2​O ⇌ Al(OH)²⁺ + H3​O⁺

K_a​ = 4.3 × 10⁻⁷

pK_a​ = 6.37

Sodium Acetate (Na+CH3COO-)

H2​O + CH3​COOH ⇌ CH3​COO− + H3​O+

Ka ​= 1.8×10⁻⁵

pK_a ​= 4.74

Sodium Carbonate (Na2CO3)

H2​O + H2​CO3​ ⇌ HCO3− ​+ H3​O+

Ka ​= 5.6 × 10⁻¹¹

pK_a = 10.5

Additional Notes

• Kb and pKb for ammonia:
• NH3​ + H2​O ⇌ NH4​OH + OH−
• K_b​=1.76×10⁻⁵
• pK_b = 4.75