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Calculate Ph of 0.1 Hcn Solution

Reviewed by Calculator Editorial Team

Calculating the pH of a hydrogen cyanide (HCN) solution is essential for understanding its acidity level. This guide explains the process, provides a calculator, and offers practical insights for chemistry students and professionals.

Introduction

Hydrogen cyanide (HCN) is a weak acid that dissociates in water to form the cyanide ion (CN⁻) and a hydrogen ion (H⁺). The pH of an HCN solution depends on its concentration and the extent of dissociation. This calculator helps determine the pH of a 0.1 molar HCN solution.

The pH scale ranges from 0 to 14, with values below 7 indicating acidity. A 0.1 M HCN solution is slightly acidic, with a pH typically between 6 and 7.

Calculation Method

The pH of an HCN solution can be calculated using the following steps:

  1. Determine the concentration of HCN in moles per liter (M).
  2. Calculate the equilibrium concentration of H⁺ ions using the dissociation constant (Kₐ) of HCN.
  3. Convert the H⁺ concentration to pH using the formula: pH = -log[H⁺].

Key Formula

The dissociation of HCN can be represented as:

HCN ⇌ H⁺ + CN⁻

The dissociation constant (Kₐ) for HCN is approximately 4.9 × 10⁻¹⁰ at 25°C.

The exact calculation involves solving the quadratic equation derived from the dissociation equilibrium.

Example Calculation

For a 0.1 M HCN solution:

  1. Assume the initial concentration of HCN is 0.1 M.
  2. Let x be the concentration of H⁺ ions that dissociate.
  3. The equilibrium concentrations are:
    • HCN: 0.1 - x M
    • H⁺: x M
    • CN⁻: x M
  4. The equilibrium expression is: Kₐ = [H⁺][CN⁻]/[HCN] = x²/(0.1 - x) ≈ 4.9 × 10⁻¹⁰.
  5. Solving this equation gives x ≈ 7.0 × 10⁻⁶ M.
  6. Therefore, pH = -log(7.0 × 10⁻⁶) ≈ 5.15.

The actual pH may vary slightly due to temperature and other factors, but this calculation provides a good approximation.

Interpretation

A pH of approximately 5.15 for a 0.1 M HCN solution indicates a weak acid. This means:

  • The solution is mildly acidic.
  • It will react with bases to form a buffer solution.
  • It should be handled with care due to the toxicity of HCN.

For comparison, a 0.1 M solution of a strong acid like HCl would have a much lower pH (around 1).

FAQ

Why is the pH of a 0.1 M HCN solution not exactly 5.15?

The pH may vary slightly due to temperature changes, impurities in the solution, or deviations from the ideal dissociation constant value. The calculation provides a close approximation.

How does the pH change with concentration?

As the concentration of HCN increases, the pH decreases because more H⁺ ions are released. For example, a 1 M HCN solution would have a lower pH than a 0.1 M solution.

Is HCN solution safe to handle?

HCN is highly toxic and should be handled with appropriate protective equipment. Always follow safety guidelines when working with cyanide compounds.