Question

Write a balanced net ionic equation to explain the observed pH for each of the solutions tested (Ionic Equilibria, pH, Indicators and Buffers Lab).

Acetic acid, HC2H3O2, pH 4.6
Aluminum chloride, AlCl3, pH 2.2
Ammonium chloride, NH4Cl, pH 4.3
Aqueous ammonia, NH3, pH 8.4
Boric acid, H3BO3, pH 4.8
Borax, Na2B4O7, pH 8.1
Citric acid, C6H8O7, pH 4.6
Hydrochloric acid, HCl, pH 4.5
Sodium acetate, NaC2H3O2, pH 6.0
Sodium carbonate, Na2CO3, pH 9.6
Sodium hydrogen carbonate, NaHCO3 pH 8.7
Sodium hydroxide, NaOH, pH 8.7

Answer 1

1. CH₃COOH (aq) + H₂O (l) <---------------------> CH₃COO^- (aq) + H₃O⁺ (aq)

Since, CH₃COOH is a weak acid, therefore its pH would be less than 7 i.e. 4.6

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2. AlCl₃ (aq) + 3 H₂O (l) <---------------------> Al(OH)₃ (aq) + 3 HCl(aq)

Ionic equation is :  Al³⁺ (aq) + 3 Cl^- (aq) + 3 H₂O (l) <------------> Al(OH)₃ (aq) + 3 H⁺ (aq) + 3 Cl^-(aq)

Net Ionic equation is :  Al³⁺ (aq) + 3 H₂O (l) <------------> Al(OH)₃ (aq) + 3 H⁺ (aq)

Since, AlCl₃ is a salt of weak base i.e. Al(OH)₃ and strong base i.e. HCl, therefore its pH would be less than 7 i.e. 2.2

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3. NH₄Cl (aq) + H₂O (l) <---------------------> NH₃ (aq) + H₂O (l) + HCl(aq)

Ionic equation is : NH₄⁺(aq) + Cl^- (aq) + H₂O (l) <-----------------> NH₃ (aq) + H₂O (l) + H⁺ (aq) + Cl^- (aq)

Net Ionic equation is :  NH₄⁺(aq) <-----------------> NH₃ (aq) + H⁺ (aq)

Since, NH₄Cl is a salt of weak base i.e. NH₃ and strong base i.e. HCl, therefore its pH would be less than 7 i.e. 4.3

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4. NH₃ (aq) + H₂O (l) <---------------------> NH₄⁺ (aq) + OH^- (aq)

Net Ionic equation is : NH₃ (aq) + H₂O (l) <---------------------> NH₄⁺ (aq) + OH^- (aq)

Since, NH₃ is a base and gives OH^- with water, so its pH will be greater than 7.