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Calculate The Ph of A 0.42 M Nh4cl Solution

Reviewed by Calculator Editorial Team

This guide explains how to calculate the pH of a 0.42 M NH4Cl (ammonium chloride) solution, including the formula, assumptions, and interpretation of results.

How to Calculate the pH of NH4Cl Solution

The pH of an NH4Cl solution can be calculated using the Henderson-Hasselbalch equation, which relates the pH to the concentration of a weak acid and its conjugate base. NH4Cl dissociates in water to form NH3 (a weak base) and HCl (a strong acid).

Key Assumption: The solution is at 25°C and the dissociation constant (Ka) for NH4+ is 5.6 × 10⁻¹⁰.

Steps to Calculate pH

  1. Determine the concentration of NH4+ (same as NH4Cl concentration since it fully dissociates).
  2. Calculate the concentration of NH3 using the dissociation constant.
  3. Apply the Henderson-Hasselbalch equation to find pH.

The pH Calculation Formula

The pH of an NH4Cl solution is calculated using the Henderson-Hasselbalch equation:

pH = pKa + log([NH3]/[NH4+])

Where:

  • pKa = -log(Ka) = -log(5.6 × 10⁻¹⁰) ≈ 9.25
  • [NH4+] = concentration of NH4+ (same as NH4Cl concentration)
  • [NH3] = concentration of NH3 (calculated from dissociation)

The concentration of NH3 can be found using the dissociation constant (Ka):

[NH3] = Ka × [NH4+] / (Ka + [NH4+])

Worked Example

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

  1. Given [NH4Cl] = 0.42 M, [NH4+] = 0.42 M.
  2. Calculate [NH3] using Ka = 5.6 × 10⁻¹⁰:

    [NH3] = (5.6 × 10⁻¹⁰ × 0.42) / (5.6 × 10⁻¹⁰ + 0.42) ≈ 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000