Calculate The Ph of A 0.42 M Nh4cl Solution.

Introduction

Ammonium chloride (NH4Cl) is a weak electrolyte that dissociates partially in water, forming NH3 (ammonia) and Cl- (chloride) ions. The pH of the solution depends on the concentration of these species and their equilibrium constants.

This calculation is based on the following assumptions:

• The solution is at 25°C (standard temperature)
• The dissociation of NH4Cl is complete (Kb for NH3 is known)
• No other reactions or impurities affect the pH

pH Calculation Formula

The pH of a solution containing NH4Cl can be calculated using the following steps:

1. Determine the concentration of NH3 formed from NH4Cl dissociation
2. Calculate the pH using the NH3 concentration and its base dissociation constant (Kb)

The calculation assumes that the concentration of NH3 equals the concentration of NH4+ at equilibrium, and uses the base dissociation constant of ammonia (NH3).

Worked Example

Let's calculate the pH of a 0.42 M NH4Cl solution step by step:

1. Initial concentration of NH4+ = 0.42 M
2. At equilibrium, [NH3] = [NH4+] = 0.42 M
3. Calculate pOH:
   pOH = pKb + log([NH3]/[NH4+]) pOH = -log(1.8 × 10⁻⁵) + log(1) pOH ≈ 4.747 + 0 = 4.747
4. Calculate pH:
   pH = 14 - pOH = 14 - 4.747 = 9.253

The pH of a 0.42 M NH4Cl solution is approximately 9.25.

Interpreting Results

A pH of 9.25 indicates a basic solution. This is expected because NH4Cl dissociates to form NH3, which is a weak base. The solution is more basic than pure water (pH 7) but less basic than strong bases like NaOH.