Calculate Ph of 0.1 Nacn Solution

Introduction

Sodium cyanide (NaCN) is a highly toxic compound that dissociates completely in water to form sodium ions (Na⁺) and cyanide ions (CN⁻). The cyanide ion (CN⁻) is a weak base, meaning it can accept protons (H⁺) from water molecules, forming the cyanate ion (OCN⁻) and a hydronium ion (H₃O⁺).

The pH of a solution can be calculated using the concentration of the hydronium ions (H₃O⁺) or the hydroxide ions (OH⁻). For a solution of NaCN, the pH is determined by the concentration of the OH⁻ ions formed from the dissociation of CN⁻.

pH Calculation Formula

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

For a solution of NaCN, the concentration of H₃O⁺ can be determined from the concentration of OH⁻, which is related to the concentration of CN⁻ by the equilibrium reaction:

The equilibrium constant (Kb) for this reaction is 4.9 × 10⁻¹⁰. Using this value, we can calculate the concentration of OH⁻ and then determine the pH.

Worked Example

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

1. First, determine the concentration of CN⁻ ions. Since NaCN is a strong electrolyte, it dissociates completely, so [CN⁻] = 0.1 M.
2. Calculate the concentration of OH⁻ using the equilibrium constant (Kb = 4.9 × 10⁻¹⁰):
[OH⁻] = √(Kb × [CN⁻]) = √(4.9 × 10⁻¹⁰ × 0.1) ≈ 7.0 × 10⁻⁶ M
3. Calculate the concentration of H₃O⁺:
[H₃O⁺] = 10⁻¹⁴ / [OH⁻] ≈ 10⁻¹⁴ / 7.0 × 10⁻⁶ ≈ 1.4 × 10⁻⁹ M
4. Finally, calculate the pH:
pH = -log[H₃O⁺] ≈ -log(1.4 × 10⁻⁹) ≈ 8.85

Therefore, the pH of a 0.1 M NaCN solution is approximately 8.85, indicating a basic solution.

Interpreting Results

The pH of a 0.1 M NaCN solution is approximately 8.85, which is significantly above 7. This indicates that the solution is basic. The basicity is due to the formation of hydroxide ions (OH⁻) from the dissociation of cyanide ions (CN⁻).

It's important to note that NaCN is highly toxic and should be handled with extreme caution. Proper safety measures, including personal protective equipment and proper disposal procedures, must be followed when working with this compound.