Calculate Ph of A 0.021 M Nacn Solution

Introduction

The pH of a solution is a measure of its acidity or basicity. For a 0.021 M NaCN solution, we can calculate the pH using the dissociation constant of cyanide (CN⁻).

Sodium cyanide (NaCN) is a strong electrolyte that dissociates completely in water, producing Na⁺ and CN⁻ ions. The CN⁻ ion is a weak base that can accept a proton to form HCN, which is a weak acid.

pH Calculation Formula

The pH of a solution containing a weak base can be calculated using the Henderson-Hasselbalch equation:

For a NaCN solution, we can rearrange this equation to calculate the pH based on the concentration of CN⁻.

The dissociation constant (K_a) for HCN is approximately 4.9 × 10⁻¹⁰, so pK_a = -log₁₀(4.9 × 10⁻¹⁰) ≈ 9.31.

Worked Example

Let's calculate the pH of a 0.021 M NaCN solution.

1. Determine the concentration of CN⁻: Since NaCN is a strong electrolyte, [CN⁻] = 0.021 M.
2. Assume the concentration of HCN is negligible (x ≈ 0) for a dilute solution.
3. Plug the values into the equation:

   pH = 9.31 + log₁₀(0.021/0) → ∞

4. Since the denominator approaches zero, the pH becomes very high, indicating a strongly basic solution.

Interpreting Results

The calculated pH of a 0.021 M NaCN solution is approximately 14, indicating a strongly basic solution. This is expected because cyanide is a weak base that can accept protons to form HCN.

In practical terms, this means the solution would feel slippery and would react strongly with acids. Always handle NaCN solutions with caution due to their high pH and potential toxicity.