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Calculate Ph of A 0.1m Nacn Solution

Reviewed by Calculator Editorial Team

This guide explains how to calculate the pH of a 0.1 molar sodium cyanide (NaCN) solution using the Henderson-Hasselbalch equation. We'll cover the chemistry behind cyanide solutions, provide a step-by-step calculation method, and discuss how to interpret the results.

Introduction

Sodium cyanide (NaCN) is a strong electrolyte that dissociates completely in water to form sodium ions (Na⁺) and cyanide ions (CN⁻). The cyanide ion (HCN) can act as a weak acid in aqueous solution, donating a proton to form the cyanide ion (CN⁻).

The pH of a NaCN solution depends on the concentration of the cyanide ion and the equilibrium between HCN and CN⁻. For dilute solutions, the pH can be calculated using the Henderson-Hasselbalch equation.

Calculation Method

The pH of a 0.1M NaCN solution can be calculated using the following steps:

  1. Determine the concentration of the cyanide ion (CN⁻). Since NaCN is a strong electrolyte, the concentration of CN⁻ is equal to the concentration of NaCN.
  2. Use the Henderson-Hasselbalch equation to calculate the pH:
pH = pKa + log([CN⁻]/[HCN])

Where:

  • pKa is the acid dissociation constant for HCN (-9.31 at 25°C)
  • [CN⁻] is the concentration of the cyanide ion
  • [HCN] is the concentration of the cyanide acid

For a 0.1M NaCN solution, [CN⁻] = 0.1M and [HCN] is negligible at this concentration, so the equation simplifies to:

pH = pKa + log([CN⁻]) = -9.31 + log(0.1)

Example Calculation

Let's calculate the pH of a 0.1M NaCN solution:

  1. Given: [NaCN] = 0.1M (strong electrolyte, so [CN⁻] = 0.1M)
  2. pKa for HCN = -9.31
  3. Calculate log(0.1) = -1
  4. pH = -9.31 + (-1) = -10.31

The calculated pH of a 0.1M NaCN solution is -10.31. This extremely low pH indicates the solution is highly acidic due to the strong acid properties of HCN.

Note: The negative pH value indicates the solution is extremely acidic. In practice, such solutions are highly toxic and require careful handling.

Interpreting Results

The pH calculation for a 0.1M NaCN solution provides several important insights:

  • The negative pH value (-10.31) indicates the solution is extremely acidic
  • The high acidity comes from the dissociation of HCN
  • The solution contains significant amounts of both HCN and CN⁻ ions
  • The pH is much lower than typical aqueous solutions

This calculation is important for understanding the behavior of cyanide solutions in chemical processes and environmental applications.

Frequently Asked Questions

Why is the pH of NaCN solution negative?
The negative pH indicates the solution is extremely acidic. This occurs because HCN is a very strong acid with a pKa of -9.31, resulting in a pH lower than 0 for concentrated solutions.
How does temperature affect the pH of NaCN solution?
The pKa of HCN changes with temperature. At higher temperatures, the pKa becomes less negative, resulting in a higher pH. Our calculation assumes 25°C, which is the standard temperature for pKa values.
Is NaCN solution safe to handle?
No, NaCN solutions are highly toxic. They should only be handled by trained professionals in controlled environments with proper safety equipment.