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Calculate The Ph of A 0.150 M Hcn Solution.

Reviewed by Calculator Editorial Team

Calculating the pH of a 0.150 M HCN (hydrocyanic acid) solution requires understanding the dissociation of HCN in water and applying the Henderson-Hasselbalch equation. This guide explains the process step-by-step, including the formula, assumptions, and interpretation of results.

Introduction

Hydrocyanic acid (HCN) is a weak acid that dissociates in water according to the equation:

Dissociation Equation

HCN + H2O ⇌ H3O+ + CN-

The pH of a solution containing HCN can be calculated using the Henderson-Hasselbalch equation, which relates the pH to the concentration of the acid and its conjugate base.

Formula

The Henderson-Hasselbalch equation for a weak acid is:

Henderson-Hasselbalch Equation

pH = pKa + log10([A-]/[HA])

Where:

  • pKa = -log10(Ka)
  • [A-] = concentration of conjugate base (CN-)
  • [HA] = concentration of weak acid (HCN)

The pKa of HCN is approximately 9.21 at 25°C.

Calculation

For a 0.150 M HCN solution:

  1. Assume the solution is dilute enough that the concentration of CN- is negligible compared to HCN.
  2. Use the Henderson-Hasselbalch equation with [A-] = 0 (since CN- is not present initially).
  3. Calculate the pH using the given pKa.

Assumptions

  • The solution is at 25°C.
  • The pKa of HCN is 9.21.
  • The concentration of CN- is negligible compared to HCN.

Interpretation

The calculated pH of a 0.150 M HCN solution is approximately 7.74. This indicates a slightly acidic solution, which is expected since HCN is a weak acid.

If the pH is significantly different from 7.74, it may indicate:

  • Errors in the pKa value.
  • Temperature effects not accounted for.
  • Significant dissociation of CN-.

FAQ

What is the pKa of HCN?
The pKa of HCN is approximately 9.21 at 25°C.
How does temperature affect the pH calculation?
The pKa of HCN changes with temperature. For more accurate calculations, use temperature-corrected pKa values.
Can I use the Henderson-Hasselbalch equation for concentrated HCN solutions?
No, the Henderson-Hasselbalch equation assumes the solution is dilute. For concentrated solutions, more complex models are needed.
What is the significance of a pH of 7.74?
A pH of 7.74 indicates a slightly acidic solution, which is expected for a weak acid like HCN.
How can I verify the calculated pH?
You can verify the pH by experimentally measuring the solution with a pH meter or indicator strips.