Calculate The Ph of A 0.250 M Hcn Solution.

Introduction

The pH of a solution is a measure of its acidity or alkalinity. For a 0.250 M HCN solution, we need to consider the dissociation of HCN in water to calculate the resulting pH.

HCN is a weak acid that dissociates partially in water according to the following equilibrium:

The pH calculation involves determining the concentration of hydronium ions (H₃O⁺) in the solution, which is then used to calculate the pH.

pH Calculation Formula

The pH of a solution is calculated using the following formula:

For a weak acid like HCN, we first need to calculate the concentration of H₃O⁺ using the dissociation constant (Kₐ) of HCN.

Once we have [H₃O⁺], we can calculate the pH using the formula above.

Worked Example

Let's calculate the pH of a 0.250 M HCN solution step by step.

1. Identify the dissociation constant of HCN: Kₐ = 5.6 × 10⁻¹⁰
2. Calculate the concentration of H₃O⁺:

   [H₃O⁺] = √(5.6 × 10⁻¹⁰ × 0.250) = √(1.4 × 10⁻¹⁰) ≈ 1.18 × 10⁻⁵ M

3. Calculate the pH:

   pH = -log(1.18 × 10⁻⁵) ≈ 4.93

The pH of a 0.250 M HCN solution is approximately 4.93, indicating it is a slightly acidic solution.

Interpreting Results

The pH value of 4.93 for a 0.250 M HCN solution indicates:

• The solution is slightly acidic (pH < 7)
• It's less acidic than pure water (pH 7) but more acidic than a typical household cleaner (pH 10)
• The HCN has partially dissociated, producing some hydronium ions