Answer
🧪 Hydroxide Ion Concentration from Hydronium Ion Concentration
📘 Key Concept: Water Ionization Constant
At 25°C, water undergoes self-ionization as follows:
H2O (l) ⇌ H3O+ (aq) + OH– (aq)
The ion-product constant for water is:
Kw = [H3O+][OH–] = 1.0 × 10-14
✏️ Step-by-Step Calculation
Given: [H3O+] = 3.5 × 10-4 M
Use the expression:
Use the expression:
[OH–] = Kw / [H3O+]
Substituting the known values:
[OH–] = (1.0 × 10-14) / (3.5 × 10-4)
Perform the division:
[OH–] = 2.86 × 10-11 M
✅ Final Answer
[OH–] = 2.86 × 10-11 M
🔍 Explanation Summary
The ion-product constant Kw at 25°C ensures that the product of the concentrations of H3O+ and OH– remains constant at 1.0 × 10-14.
When [H3O+] increases, [OH–] must decrease to maintain this equilibrium. Here, a relatively large hydronium ion concentration results in a low hydroxide ion concentration, confirming the solution is acidic.
When [H3O+] increases, [OH–] must decrease to maintain this equilibrium. Here, a relatively large hydronium ion concentration results in a low hydroxide ion concentration, confirming the solution is acidic.