Calculate Ph of 0.1m Nh3

Introduction

The pH of a solution is a measure of its acidity or alkalinity. For ammonia solutions, the pH is determined by the equilibrium between the undissociated ammonia (NH3) and its conjugate acid (NH4+).

Ammonia is a weak base with a dissociation constant (Kb) of 1.8 × 10⁻⁵ at 25°C. The pH of a dilute ammonia solution can be calculated using the Henderson-Hasselbalch equation, which relates the pH to the concentration of the weak base and its conjugate acid.

Formula

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

Where:

• pKb is the negative logarithm of the base dissociation constant (Kb)
• [NH3] is the concentration of ammonia
• [NH4+] is the concentration of ammonium ions

For a dilute solution, the concentration of NH4+ is approximately equal to the initial concentration of NH3 because very little NH3 dissociates.

Calculation

To calculate the pH of a 0.1 molar ammonia solution:

1. Determine the pKb value for ammonia. At 25°C, pKb = -log(1.8 × 10⁻⁵) ≈ 4.74.
2. Assume that the concentration of NH4+ is equal to the initial concentration of NH3 (0.1 M) because the solution is dilute.
3. Plug the values into the Henderson-Hasselbalch equation:

   pH = 4.74 + log₁₀([NH3]/[NH4+]) = 4.74 + log₁₀(0.1/0.1) = 4.74 + log₁₀(1) = 4.74 + 0 = 4.74

The pH of a 0.1 molar ammonia solution is approximately 4.74.

Example

Let's calculate the pH of a 0.1 molar ammonia solution step-by-step:

1. Given: [NH3] = 0.1 M, [NH4+] ≈ 0.1 M (for dilute solutions)
2. pKb for NH3 at 25°C = 4.74
3. Using the Henderson-Hasselbalch equation:

   pH = 4.74 + log₁₀(0.1/0.1) = 4.74 + 0 = 4.74

The calculated pH is 4.74, which indicates a slightly alkaline solution.